[llvm] [SLP] Loop aware cost model/tree building (PR #150450)

[Alexander Kornienko via llvm-commits]

alexfh wrote:

@alexey-bataev heads up, this PR (specifically, 6261cb4487f153c599a040d7a77524561b520240) introduces runtime performance regressions on x86 in some of our internal microbenchmarks.

https://github.com/llvm/llvm-project/pull/150450