[llvm] Adding the support for vectorization of loops with load based tripcount (PR #209390)

[Manish Srivastava via llvm-commits]

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mk-srivastava wrote:

Seems logical, will divide the whole PR into 3 separate PRs. 
1. Will be for SCEV change
2. Will be for LAA change
3. Final will be for LoopVectorize change

Will keep this PR up though to showcase the overall intention and actual working I am hoping to get. 

https://github.com/llvm/llvm-project/pull/209390