[llvm] [IndVarSimplify] Fix Masking Issue by Adding nsw/nuw Flags to Trunc Instruction (PR #150179)

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buggfg wrote:

> Thanks for updating the PR, I'm taking a closer look at it now. In the meantime though is it possible to add any tests to `llvm/test/Transforms/IndVarSimplify/lftr-*.ll` to illustrate the effect of this change more clearly?

Your suggestion is great! I'll prepare the test

https://github.com/llvm/llvm-project/pull/150179