[X86] Add support for {vex2}, {vex3}, and {evex} to the assembler to match...

[X86] Add support for {vex2}, {vex3}, and {evex} to the assembler to match gas. Use {evex} to improve the one our 32-bit AVX512 tests.

These can be used to force the encoding used for instructions.

{vex2} will fail if the instruction is not VEX encoded, but otherwise won't do anything since we prefer vex2 when possible. Might need to skip use of the _REV MOV instructions for this too, but I haven't done that yet.

{vex3} will force the instruction to use the 3 byte VEX encoding or fail if there is no VEX form.

{evex} will force the instruction to use the EVEX version or fail if there is no EVEX version.

Differential Revision: https://reviews.llvm.org/D59266

llvm-svn: 358029