[X86][MC] Support decoding of EGPR for APX (#72102)

https://github.com/llvm/llvm-project/pull/70958 adds registers R16-R31
(EGPR), this patch


1. Supports decoding of EGPR for instruction w/ REX2 prefix
2. Supports decoding of EGPR for instruction w/ EVEX prefix

For simplicity's sake,  we 
1.  Simulate the REX prefix w/ the 1st payload of REX2
2.  Simulate the REX2 prefix w/ the 2nd and 3rd payloads of EVEX

RFC:

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

Explanations for some changes:
1. invalid-EVEX-R2.txt is deleted b/c `0x62 0xe1 0xff 0x08 0x79 0xc0` is
valid and decoded to `vcvtsd2usi %xmm0, %r16` now.
2. One line in x86-64-err.txt is removed b/c APX relaxes the limitation
of the 1st and 2nd payloads of EVEX prefix, so the error message changes