[llvm] [InlineCost] Cache collectEphemeralValues() to save compile time (PR #130210)

[via llvm-commits]

vporpo wrote:

This solves the same problem as https://github.com/llvm/llvm-project/pull/129279 but using caching instead of introducing a hard limit.
I am not super confident that the cache won't need to be invalidated, but that's what I saw in my (limited) testing.

https://github.com/llvm/llvm-project/pull/130210