[llvm] Allow MIN/MAX signedness flip when operands are known-negative (PR #174469)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon requested changes to this pull request.

don't bother with a new test file - just add the tests to the existing combine-smax.ll, combine-smin.ll etc. the existing nosignbit tests should already be there

https://github.com/llvm/llvm-project/pull/174469