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

[via llvm-commits]

goldsteinn wrote:

> > > 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.
> > 
> > 
> > This matches `m_NonZeroFp` (Which I guess I could have just used instead).
> 
> Made default no-undef + tests.

If you would prefer, I can just drop the whole `m_NonZero` stuff and use `m_NonZeroFP`.

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