[llvm] [LoopVectorize] Compute BlockFrequencyInfo eagerly to fix assertion failure (PR #215237)

[Karthika Devi C via llvm-commits]

kartcq wrote:

Thanks for the review @fhahn 
I have resolved the ones regarding the test.
For the concern regarding compilation time, I have requested @nikic for my fork to get picked up by compile-time-tracker.

https://github.com/llvm/llvm-project/pull/215237