[LoopVectorize] Preserve CFG analyses if CFG wasn't modified

One of transforms the loop vectorizer makes is LCSSA formation. In some cases it
is the only transform it makes. We should not drop CFG analyzes if only LCSSA was
formed and no actual CFG changes was made.

We should think of expanding this logic to other passes as well, and maybe make
it a part of PM framework.

Reviewed By: Florian Hahn
Differential Revision: https://reviews.llvm.org/D78360