[llvm] [SCEV] Fix cache poisoning from depth-limited truncate expressions (PR #172234)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

Please reference the issue you are fixing. I remember there being one for this.

I initially thought that this change may cause compile-time issues for the pathological cases the depth limit is intended to address, but after considering more carefully, I don't think it will really affect them. The cache will only be hit when using the argument after the recursion has already happened. (We have a different cache that will use the original argument, but that's not used for trunc.)

https://github.com/llvm/llvm-project/pull/172234