[AMDGPU] Fix operand definitions for atomic scalar memory instructions. (#71799)

CPol and CPol_GLC1 operand classes have identical predicates, which
means AsmParser cannot differentiate between the RTN and non-RTN
variants of the instructions. When it currently selects the wrong
instruction, a hack in cvtSMEMAtomic() corrects the op-code. Using the
new predicated-value operands makes this hack and the whole conversion
function not needed.

Other uses of CPol_GLC1 operands are to be addressed separately.

Resolves about half of the remaining ~1000 pairs of ambiguous
instructions.

Part of <https://github.com/llvm/llvm-project/issues/69256>.