Merge HasVEXPrefix/HasEVEXPrefix/HasXOPPrefix into a 2-bit 'encoding' field in TSFlags.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200624 91177308-0d34-0410-b5e6-96231b3b80d8
Simplify some code since VEX and EVEX instructions never have HasOpSizePrefix.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200625 91177308-0d34-0410-b5e6-96231b3b80d8