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

[Luke Lau via llvm-commits]

lukel97 wrote:

> How does compile-time for the new variant look like?

Somehow even worse than the baseline: https://llvm-compile-time-tracker.com/compare.php?from=aff98e4be05a1060e489ce62a88ee0ff365e571a&to=7a0bf19b515a555f0b80c55e8d700dfcc8e88ff4&stat=instructions%3Au

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