[SCEV] Cache ZExt SCEV expressions.

When creating SCEV expressions for ZExt, there's quite a bit of
reasoning done and in many places the reasoning in turn will try to
create new SCEVs for other ZExts.

This can have a huge compile-time impact. The attached test from #58402
takes an excessive amount of compile time; without the patch, the test
doesn't complete in 1500+ seconds, but with the patch it completes in 1
second.

To speed up this case, cache created ZExt expressions for given (SCEV, Ty) pairs.
Caching just ZExts is relatively straight-forward, but it might make
sense to extend it to other expressions in the future.

This has a slight positive impact on CTMark:
* O3: -0.03%
* ReleaseThinLTO: -0.03%
* ReleaseLTO-g: 0.00%

https://llvm-compile-time-tracker.com/compare.php?from=bf9de7464946c65f488fe86ea61bfdecb8c654c1&to=5ac0108553992fb3d58bc27b1518e8cf06658a32&stat=instructions:u

The patch also improves compile-time for some internal real-world workloads
where time spent in SCEV goes from ~300 seconds to ~3 seconds.

There are a few cases where computing & caching the result earlier may
return more pessimistic results, but the compile-time savings seem to
outweigh that.

Fixes #58402.

Reviewed By: mkazantsev

Differential Revision: https://reviews.llvm.org/D137505