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

[Jay Foad via llvm-commits]

jayfoad wrote:

> A miscompile is caused by ordering issues: #78383 Nonetheless, I would expect UNDEF and catching misbeviour be executed/registered before optimizations.

I think it's wrong to say that ordering issues _cause_ that problem. One of the combines is broken - that's what causes the problem. Ordering issues just happen to _hide_ the problem in some cases.

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