[IR] Add support for trunc's nuw/nsw flags in `copyIRFlags` (#89353)

This patch fixes
https://github.com/llvm/llvm-project/pull/85592#issuecomment-2065865020.

I found this while fixing flag propagation in my
["vectorizer"](https://github.com/dtcxzyw/llvm-codegen-benchmark/blob/main/vectorize.cpp).