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

[Thorsten Schütt via llvm-commits]

tschuett wrote:

I believe that we should move to a visitOpcode model like the Dag combiner eventually. If this combine is registered before, `constant_fold_fp_binop`, then we get a `fneg`.  Otherwise, we may have gotten a constant for `fmul 1000,0 * -1`.

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