Add support for PointerLikeRegClass to FixedLenDecoderEmitter

This is more prep for adding the PowerPC disassembler. FixedLenDecoderEmitter
should recognize PointerLikeRegClass operands as register types, and generate
register-like decoding calls instead of treating them like immediates.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197680 91177308-0d34-0410-b5e6-96231b3b80d8