[X86][NFC] Avoid uselss iterations when emitting EVEX compression table

BTW, we relax the condition for EVEX compression from
ST.hasAVX512() to ST.hasEGPR() || ST.hasAVX512(). It does not have any
effect now b/c no APX instruction is in the EVEX compression table so
far.

This patch is to extract NFC in #77065 into a separate commit.