[llvm] [SCCP] Refine trunc with nsw/nuw flags (PR #87926)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> BTW, as adding poison-generating flags blocks `logical and/or -> bitwise and/or`, should we take context information into account in `canCreateUndefOrPoison`? @nikic

Ping. @nikic 

https://github.com/llvm/llvm-project/pull/87926