[X86][MC] Support Enc/Dec for EGPR for promoted VMX instructions (#74434)

R16-R31 was added into GPRs in
https://github.com/llvm/llvm-project/pull/70958,
This patch supports the encoding/decoding for promoted VMX instructions
in EVEX space.

RFC:
https://discourse.llvm.org/t/rfc-design-for-apx-feature-egpr-and-ndd-support/73031/4