Update comments.

Merge the REX2 changes into the original legacy emit path

bug fix: Set REX2.W with correct mask code.

register encoding and prefix emitting logics.

Add REX2 prefix emit logic

bug fixes

Add Stress mode for REX2 encoding and some bug fixes

resolve comments:
1. add assertion check for UD opcodes.
2. add checks for EGPRs.

Add REX2 to emitOutputAM, and let LEA to be REX2 compatible.

Add REX2.X encoding for SIB byte

But fixes: add REX2 prefix on the path in RI where MOV is specially handled.

Enable REX2 encoding for `movups`

fixed bugs in REX2 prefix emitting logic when working with map 1 instructions, and enabled REX2 for POPCNT

legacy map index-er

bug fixes

some clean-up

Adding initial APX unit testing path.

Adding a coredistools dll that has LLVM APX disasm capability.

It must be coppied into a CORE_ROOT manually.

clean up work for REX2

narrow the REX2 scope to `sub` only

some clean up based on the comments.

bug fix

resolve comment

 - SV path is mostly for debugging purposes

Added encoding unit tests for instructions with immediates

Code refactoring: AddX86PrefixIfNeeded.

… missing in JIT, may indicate these instructions are not being used in JIT, drop them for now.

…lled before adding any prefix.

Refactor REX2 encoding stress logics.

(this will have side effect that the estimated code will go up and mismatch with actual code size.)