Revert "[FuncSpec] Replace LoopInfo with BlockFrequencyInfo"

As reported on https://reviews.llvm.org/D150375#4367861 and
following, this change causes PDT invalidation issues. Revert
it and dependent commits.

This reverts commit 0524534d.
This reverts commit ced90d1f.
This reverts commit 9f992cc9.
This reverts commit 1b123204.