[InstSimplify] try harder to fold fmul with 0.0 operand

https://alive2.llvm.org/ce/z/oShzr3

This was noted as a missing fold in D134876 (with additional
examples based on issue #58046).

I'm assuming that fmul with a zero operand is rare enough
that the use of ValueTracking will not noticeably increase
compile-time.

This adjusts a PowerPC codegen test that was added with D88388
because it would get folded away and no longer provide coverage
for the bug fix.