[DAGCombine][PowerPC] Simplify nabs by using legal `smin` operation

Convert `0 - abs(x)` to `smin (x, -x)` if `smin` is a legal operation.

Verification: https://alive2.llvm.org/ce/z/vpquFR

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D92637