[X86] Add OpSize32 to XBEGIN_4. Add XBEGIN_2 with OpSize16.

[X86] Add OpSize32 to XBEGIN_4. Add XBEGIN_2 with OpSize16.

Requires new AsmParserOperand types that detect 16-bit and 32/64-bit mode so that we choose the right instruction based on default sizing without predicates. This is necessary since predicates mess up the disassembler table building.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225256 91177308-0d34-0410-b5e6-96231b3b80d8