[LoopVectorize] Only fetch BFI if profile summary available

[LoopVectorize] Only fetch BFI if profile summary available

BlockFrequencyInfo should generally only be fetched in PGO builds
where a PSI profile summary is available. However, LoopVectorize
was fetching it unconditionally.

This results in a small compile-time improvement for non-PGO builds.

Differential Revision: https://reviews.llvm.org/D144953