Merge x86 HasOpSizePrefix/HasOpSize16Prefix into a 2-bit OpSize field with 0 meaning no 0x66 prefix in any mode. Rename Opsize16->OpSize32 and OpSize->OpSize16. The classes now refer to their operand size rather than the mode in which they need a 0x66 prefix. Hopefully can merge REX_W into this as OpSize64.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200626 91177308-0d34-0410-b5e6-96231b3b80d8