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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Do we have no choice but to iterate over the EGPRs even for cpus that don't support them? Why does TRI::getNumRegs() always include them?

What in particular about that HandlePhysRegDef loop is causing the perf regression? Surely at the moment we're just getting as far as the `(PhysRegDef[i] || PhysRegUse[i])` check which is always false, is that part that expensive?

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