Fill some code coverage gaps in LambdaCompiler

[bartdesmet]

There are some notable gaps for non-trivial pieces of code. A few are listed below:

• EmitMethodAndAlso and EmitMethodOrElse
• EmitBranchNot, EmitBranchLogical, EmitBranchAnd, EmitBranchOr
• Some branching with statically determined null values
• Emit for hashtable switches and other code paths around emit for Switch expressions
• Constrained virtual calls
• Emit for Dynamic expressions
• Emit for Goto expressions
• Emit for Extension expressions (doesn't seem like reachable code; StackSpiller will expand these)
• Various cases with nullables and value types

[VSadov]

We need to have some way of tracking the progress of this bug. Perhaps the next step would be collecting actual items that need addressing and add them as checkboxes to the bug?