[x86] Start refactoring the comment printing logic in the MC lowering of
vector shuffles.

This is just the beginning by hoisting it into its own function and
making use of early exit to dramatically simplify the flow of the
function. I'm going to be incrementally refactoring this until it is
a bit less magical how this applies to other instructions, and I can
teach it how to dig a shuffle mask out of a register. Then I plan to
hook it up to VPERMD so we get our mask comments for it.

No functionality changed yet.
R600/SI: Enable named operand table for SALU instructions
R600/SI: Fix hardcoded and wrong operand numbers.

Also fix leftover debug printing
R600/SI: Add new helper isSGPRClassID

Move these into header since they are trivial