[Reassociate] Remove bogus assert reported in PR42349.

[Reassociate] Remove bogus assert reported in PR42349.

Also, add a FIXME for the unsafe transform on a unary FNeg. A unary FNeg can only be transformed to a FMul by -1.0 when the nnan flag is present. The unary FNeg project is a WIP, so the unsafe transformation is acceptable until that work is complete.

The bogus assert with introduced in D63445.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@363998 91177308-0d34-0410-b5e6-96231b3b80d8