[llvm] [InstSimplify] Fold value-preserving zext/sext of trunc (PR #204089)

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

RISC-V diffs LGTM,  There's a handful of loop vectorized loops where we're no longer computing the final value of an induction up front, and are instead doing it in the loop? But that's something to fix in the loop vectorizer/SCEV. 

https://github.com/llvm/llvm-project/pull/204089