[jit] Missing OP_BOX optimization

[filipnavara]

CoreCLR optimizes the following sequences that Mono does not:

• box(x).GetType()
• box(x) == null and box(x) != null
• box(this).Method() ... cannot link that, file is too big for GitHub; just look for thisObj->IsBoxedValue()

Also, Mono currently uses OP_BOX opcode only for enums which would make implementing these cases difficult.

It shows up in some microbenchmarks and it may have some implications for generic code. We should evaluate whether any of these optimizations are useful.