[InstCombine] fix profitability equation for max-of-nots transform

[InstCombine] fix profitability equation for max-of-nots transform

As the test change shows, we can increase the critical path by adding
a 'not' instruction, so make sure that we're actually removing an
instruction if we do this transform.

This transform could also cause us to miss folds of min/max pairs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@286315 91177308-0d34-0410-b5e6-96231b3b80d8