[DAG][PowerPC] Fix dropped `nsw` flag in `SimplifySetCC` by adding `doesNodeExist` helper

`SimplifySetCC` invokes `getNodeIfExists` without passing `Flags` argument and `getNodeIfExists` uses a default `SDNodeFlags` to intersect the original flags, as a consequence, flags like `nsw` is dropped. Added a new helper function `doesNodeExist` to check if a node exists without modifying its flags.

Reviewed By: #powerpc, nemanjai

Differential Revision: https://reviews.llvm.org/D89938