[SLP]Drop nsw when shl by BW-1 is converted to mul
shl nsw X, BW-1 is valid at X = -1, but mul X, INT_MIN is not nsw-safe
there. Drop nsw on the vector mul when a shl lane with shift amount
BW-1 is unified into mul during opcode interchange.
Fixes #210093
Reviewers:
Pull Request: https://github.com/llvm/llvm-project/pull/210152
