[SCEVExpander] Recompute poison-generating flags on hoisting. PR57187

Instruction being hoisted could have nuw/nsw flags inferred from the old
context, and we cannot simply move it to the new location keeping them
because we are going to introduce new uses to them that didn't exist before.

Example in https://github.com/llvm/llvm-project/issues/57187 shows how
this can produce branch by poison from initially well-defined program.

This patch forcefully recomputes poison-generating flag in the new context.

Differential Revision: https://reviews.llvm.org/D132022
Reviewed By: fhahn, nikic