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

[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.

llvm-svn: 316273