[NFC][AMDGPU] Add test for unfold-masked-merge-scalar-variablemask.ll (llvm#140093)

This enables DAGCombine to recognize and form and_not + or patterns when
hasAndNot is true, allowing better SALU codegen for masked merge idioms.
For example:
(A & (B ^ C)) ^ C → (A & B) | (~A & C)

References:
* https://reviews.llvm.org/D45563#change-EG4Z6zDTofCc
* https://bugs.llvm.org/show_bug.cgi?id=6773
* https://graphics.stanford.edu/~seander/bithacks.html#MaskedMerge

Author: harrisonGPU