[llvm] [SCEV] Support ule/sle exit counts via widening (PR #92206)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

In general, you end up with a sext/zext of some expression which still isn't analyzable... but I guess you end up with a useful result in a lot of cases, as demonstrated by the testcases.  Maybe worth explicitly noting that you expect the sign/zero-extension to simplify.

My biggest concern here is that we'll somehow end up generating code for a bunch of double-register-width math, which would be relatively expensive to compute.  Not sure how likely that is, though.

https://github.com/llvm/llvm-project/pull/92206