Investigate EETypePtr overhead

[jkotas]

Context: #225 (comment)

RuJIT is not able to optimize out the EETypePtr wrapper completely. In the example above the EETypePtr wrapper:

• Disables tailcall optimization. RyuJIT is not able to prove that the byrefs taken by EETypePtr are not passed into the worker method
• Results into less efficient register allocation and instruction selection (e.g. cmp [rcx], rdx vs. mov rax, [rcx] cmp rax, rdx)

[jkotas]

Another factor to consider is code sharing with full CoreCLR. MethodTable and EEType are 80% the same between. We need a plan how to share the code that uses these. For example, the Array.Copy impl could have been shared except this difference.

EEType is a better name. Unfortunately, MethodTable is too entrenched in CoreCLR in all code, documentation and user facing surface (e.g. DumpMT command in SOS).