[NativeAOT-LLVM] Do not create useless 'alloca's for shadow locals

Most shadow locals do not need to be materialized in allocas or LLVM registers. Account for this by maintaining an accurate reference count of them during LSSA.
Diffs (debug code, there are none in optimized code, as expected):
Summary of Code Size diffs:
(Lower is better)

Total bytes of base: 16135112
Total bytes of diff: 15594933
Total bytes of delta: -540179 (-3.35% % of base)
Average relative delta: -6.62%
    diff is an improvement
    average relative diff is an improvement

Top method regressions (percentages):
         197 ( 8.16% of base) : 6654.dasm - S_P_CoreLib_System_Globalization_CalendricalCalculationsHelper__EquationOfTime
         345 ( 5.57% of base) : 7436.dasm - S_P_TypeLoader_Internal_Runtime_TypeLoader_TypeLoaderEnvironment__CompareTypeSigs
          93 ( 5.03% of base) : 4514.dasm - S_P_CoreLib_System_TimeZoneInfo__ParseTimeOfDay
         293 ( 4.01% of base) : 4504.dasm - S_P_CoreLib_System_DateTimeFormat__TryFormatR<Char>
          68 ( 3.76% of base) : 6536.dasm - S_P_CoreLib_System_Number__TryUInt32ToBinaryStr<UInt8>
         109 ( 3.75% of base) : 1624.dasm - S_P_Reflection_Execution_Internal_Reflection_Execution_ConstraintValidator__SatisfiesConstraints
         569 ( 3.59% of base) : 4812.dasm - S_P_CoreLib_System_Number__TryParseBinaryIntegerStyle<Char__UInt64>
         569 ( 3.59% of base) : 4286.dasm - S_P_CoreLib_System_Number__TryParseBinaryIntegerStyle<Char__Int64>
          49 ( 3.34% of base) : 5937.dasm - S_P_CoreLib_System_Number__TryUInt32ToBinaryStr<Char>
         522 ( 3.32% of base) : 3900.dasm - S_P_CoreLib_System_Number__TryParseBinaryIntegerStyle<Char__UInt32>
         522 ( 3.32% of base) : 4281.dasm - S_P_CoreLib_System_Number__TryParseBinaryIntegerStyle<Char__Int32>
         526 ( 3.27% of base) : 4291.dasm - S_P_CoreLib_System_Number__TryParseBinaryIntegerStyle<Char__UInt8>
          60 ( 3.26% of base) : 6537.dasm - S_P_CoreLib_System_Number__TryInt32ToHexStr<UInt8>
         526 ( 3.26% of base) : 7448.dasm - S_P_CoreLib_System_Number__TryParseBinaryIntegerStyle<Char__UInt16>
         526 ( 3.22% of base) : 2903.dasm - S_P_CoreLib_System_Number__TryParseBinaryIntegerStyle<Char__Int8>
         526 ( 3.22% of base) : 4297.dasm - S_P_CoreLib_System_Number__TryParseBinaryIntegerStyle<Char__Int16>
          82 ( 3.11% of base) : 4502.dasm - S_P_CoreLib_System_DateTimeFormat__TryFormatu<Char>
          63 ( 2.96% of base) : 4503.dasm - S_P_CoreLib_System_DateTimeFormat__TryFormatS<Char>
          86 ( 2.85% of base) : 2129.dasm - S_P_CoreLib_System_Number__FindSection
          41 ( 2.75% of base) : 5108.dasm - S_P_CoreLib_System_Number__TryInt64ToHexStr<Char>

Top method improvements (percentages):
         -99 (-61.11% of base) : 5776.dasm - S_P_CoreLib_System_Runtime_GCStress__Initialize
       -1285 (-53.08% of base) : 2916.dasm - S_P_CoreLib_System_Runtime_RuntimeExports__RhpCalculateStackTraceWorker
         -71 (-51.82% of base) : 2929.dasm - S_P_CoreLib_Interop_Sys__Abort
         -71 (-51.82% of base) : 3960.dasm - HelloWasm_Program__NativeIntToDoubleWrongAnother
         -71 (-51.82% of base) : 3964.dasm - HelloWasm_Program__MemSetWrongAnother
         -57 (-51.82% of base) : 3316.dasm - S_P_CoreLib_System_Globalization_OrdinalCasing__get_NoCasingPage
         -57 (-49.14% of base) : 6168.dasm - S_P_CoreLib_System_GC__GetGCMemoryInfo
         -57 (-47.50% of base) : 7503.dasm - S_P_CoreLib_System_SR__get_IO_SeekAppendOverwrite
         -57 (-47.50% of base) : 5448.dasm - S_P_CoreLib_System_SR__get_Format_InvalidString
         -57 (-47.50% of base) : 6451.dasm - S_P_CoreLib_System_SR__get_Argument_InvalidSeekOrigin
         -57 (-47.50% of base) : 3716.dasm - S_P_CoreLib_System_SR__get_InvalidOperation_DateTimeParsing
         -57 (-47.50% of base) : 6085.dasm - S_P_CoreLib_System_SR__get_ArgumentOutOfRange_DateArithmetic
         -57 (-47.50% of base) : 5958.dasm - S_P_CoreLib_System_SR__get_InvalidOperation_UnknownEnumType
         -68 (-46.58% of base) : 6595.dasm - S_P_CoreLib_System_TimeZoneInfo__GetLocalTimeZoneCore
         -68 (-46.58% of base) : 3468.dasm - S_P_CoreLib_System_Globalization_CultureInfo__GetUserDefaultUICulture
         -57 (-45.24% of base) : 4330.dasm - S_P_CoreLib_System_IO_Path__GetInvalidPathChars
         -30 (-42.86% of base) : 3107.dasm - S_P_CoreLib_System_TimeZoneInfo__GetUtcStandardDisplayName
         -30 (-42.86% of base) : 3522.dasm - S_P_CoreLib_System_AppContext__GetBaseDirectoryCore
        -115 (-39.25% of base) : 4817.dasm - S_P_CoreLib_System_Runtime_InternalCalls__RhpGetGcTotalMemory
         -57 (-38.51% of base) : 3615.dasm - S_P_CoreLib_System_NotImplemented__get_ByDesign

7640 total methods with Code Size differences (7548 improved, 92 regressed)
This does not move the needle much in the extreme cases, but it's an improvement nevertheless.
dotnet / runtimelab

[NativeAOT-LLVM] Do not create useless 'alloca's for shadow locals #2495
