[AMDGPU] Allow register tuples to set asm names

This change reverts most of the previous register name generation.
The real problem is that RegisterTuple does not generate asm names.
Added optional operand to RegisterTuple. This way we can simplify
register name access and dramatically reduce the size of static
tables for the backend.

Differential Revision: https://reviews.llvm.org/D64967

llvm-svn: 366598