[X86] Move the AC_EVEX_2_VEX AsmComments enum to X86InstrInfo.h from X86InstComments.h.

X86InstComments.h is used by tools that only have the MC layer. We shouldn't be importing a file from CodeGen into this.

X86InstrInfo.h isn't a great place, but I couldn't find a better one.

llvm-svn: 327202