[llvm] [InstCombine] Fix behavior for `(fmul (sitfp x), 0)` (PR #85298)

[Nikita Popov via llvm-commits]

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

I haven't checked whether it might be safe in this context, but the m_NonZero you added here is an unacceptable footgun: It allows undef values that may refine to zero.

https://github.com/llvm/llvm-project/pull/85298