[X86] Add support for vex, vex2, vex3, and evex for MASM
For MASM syntax, the prefixes are not enclosed in braces.
The assembly code should like:
"evex vcvtps2pd xmm0, xmm1"
Differential Revision: https://reviews.llvm.org/D90441
LiuChen3 on Oct 29 2020, 8:11 PM.Authored by