[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