[llvm] [SCEV] Cache collected loop guards. NFCI (PR #116947)

[Luke Lau via llvm-commits]

lukel97 wrote:

Ah right, I had a feeling this was a bit naïve. Would there be any issue if we instead tried to collect the loop guards once in computeBackedgeTakenCount?

https://github.com/llvm/llvm-project/pull/116947