[AMDGPU] Use AddedComplexity for ret and noret atomic ops selection

This patch removes the predicate for return atomic ops and uses
AddedComplexity to distinguish its selection from its no return variant.
This will produce better matchers that doesn't unnecessarily check for
the negated predicate if the initial predicate failed. Also, it
simplifies the enabling of no return atomic ops selection in GlobalISel.

Differential Revision: https://reviews.llvm.org/D128241