[Reassociate] Don't propogate flags when creating negations

Reassociate mutated existing instructions in order to form negations
which would create additional reassociate opportunities.

This fixes PR23926.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240593 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed