[X86] Return more accurate getNumSupportedRegs() (NFC) (#71690)

https://github.com/llvm/llvm-project/pull/70222 introduced a hook to
return a more accurate number of registers supported for a specific
subtarget (rather than target). However, while x86 registers were
reordered to allow using this, the implementation currently still always
returns NUM_TARGET_REGS.

Adjust it to return a smaller number of registers depending on
availability of avx/avx512/amx.

The actual impact of this seems to be pretty small, on the order of
0.05%.