[llvm] [GlobalISel] replace right identity X * -1.0 with fneg(x) (PR #80526)

[Jay Foad via llvm-commits]

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

I don't think this is safe since fmul is an arithmetic operation that may quieten nans and flush denormals, whereas fneg is defined to just flip the sign bit.

See https://reviews.llvm.org/D109446 for the DAG equivalent.

https://github.com/llvm/llvm-project/pull/80526