Fix InvalidProgramException

[tana]

This PR solves some InvalidProgramExceptions, which is thrown when executing broken IL. Modifications are:

• Stopped using unaligned. 8 IL instruction. Because 8-byte alignment is not supported in .NET, using unaligned. 8 caused an InvalidProgramException. Because 8-byte alignment is subset of 4-byte alignment, it was safely replaced by default 4-byte alignment.
  • This problem was noted in yself's comment comment of #21.
• In i64.(shl|shr|shr_s) instructions, I added type conversion of shift amount operand. This is because IL shift instructions only support 32-bit shift amount.
• In IL of i64.clz instruction, an explicit conversion from int32 to int64 was added.
  • This is probably unnecessary in .NET Framework, but needed for Mono (at least in Unity).
  • FYI, I use dotnet-webassembly on Unity through forcibly compiling for .NET Framework 4.7.1.

[RyanLamansky]

I expect to have enough free time to package this into a release this weekend.