[PowerPC] Do not fold `cmp(d|w)` and `subf` instruction to `subf.` if `nsw` is not present

In `PPCInstrInfo::optimizeCompareInstr` we seek opportunities to fold `cmp(d|w)` and `subf` as an `subf.`. However, if `subf.` gets overflow, `cr0` can't reflect the correct order, violating the semantics of `cmp(d|w)`.

Fixed https://bugs.llvm.org/show_bug.cgi?id=47830.

Reviewed By: #powerpc, nemanjai

Differential Revision: https://reviews.llvm.org/D90156