[LTO] Enhance unified/nonunified LTO checks. (#148229)
For the PS targets, unified LTO pipeline is used as default behaviour when enabling LTO. Other targets use Distinct LTO pipeline behaviour as default behavious. Unified/nonunified LTO checks are enhanced in this PR:
1. Check that the default, unified, and non-unified behavior for asan-unified-lto.ll all match (irrespective of what the default unified/nonunified behavior is).
2. Check that the difference in behavior for 'unified' and 'full' runs is (regarding the passes) is what is expected (rather than simply that they are different).
