[SLP] Drop nsw when mul by INT_MIN is converted to shl
mul nsw X, INT_MIN is valid, but shl X, BW-1 is not nsw-safe. Drop nsw on
the vector shl when a mul lane with INT_MIN is unified into shl during
opcode interchange.
Fixes #207990
Reviewers:
Pull Request: https://github.com/llvm/llvm-project/pull/208028
