[X86] Fix disassembling of EVEX instructions to stop accidentally decoding the SIB...

[X86] Fix disassembling of EVEX instructions to stop accidentally decoding the SIB index register as an XMM/YMM/ZMM register.

This introduces a new operand type to encode the whether the index register should be XMM/YMM/ZMM. And new code to fixup the results created by readSIB.

This has the nice effect of removing a bunch of code that hard coded the name of every GATHER and SCATTER instruction to map the index type.

This fixes PR32807.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316273 91177308-0d34-0410-b5e6-96231b3b80d8