[llvm] [X86] Support EGPR (R16-R31) for APX (PR #67702)

[Shengchen Kan via llvm-commits]

KanRobert wrote:

>From the table above, we know that there are 15M extra instructions. `(PhysRegDef[i] || PhysRegUse[i])` here contributes about 2M. `HandlePhysRegDef` does not cause the regression. Here is the statistic:

Function name | extra instructions
---------------- | -------------------
__memset_avx2_unaligned_erms  | 3M
LiveVariables::runOnBlock  | 2M
LiveRegUnits::removeRegsNotPreserved | 2M
ReachingDefAnalysis::enterBasicBlock | 2M
llvm::LiveVariables::HandleRegMask | 1M
_int_malloc | 1M
other  | 4M

All the regressions are about the iteration over EGPRs. 

https://github.com/llvm/llvm-project/pull/67702