[AMDGPU] Fix useDeprecatedPositionallyEncodedOperands errors in R600.

This is a follow-on to https://reviews.llvm.org/D134073.

It renames a couple of fields to match their operands, as well as
introducing sub-operand names where required.

This change _only_ fixes the 'R600' half of the target, not the
'AMDGPU' half. Fixing the AMDGPU half will be a significantly more
difficult change (which I've not yet attempted.)

Differential Revision: https://reviews.llvm.org/D134078