[llvm] [LoadStoreVectorizer] Postprocess and merge equivalence classes (PR #121861)

[Vyacheslav Klochkov via llvm-commits]

v-klochkov wrote:

The first 2 commits in this patch are:
1) the cherry-pick of fd2f8d485df7742320317b14d49b9d808f70625c, which reported fail for 1 of tests at post-commit checks with memory-sanitizers, and was reverted after the 1st attempt to merge it.
2) The fix for memory error + extra LIT case + minor adjustments in debug printings: c57ce512d84654faf47ef59770d876f3f1dc3844

https://github.com/llvm/llvm-project/pull/121861