[llvm] [InstCombine] Do not require nsz for reassociating fmul (PR #93698)

[Joshua Cranmer via llvm-commits]

jcranmer-intel wrote:

You shouldn't need `nsz` for reassociating `fadd` either. The sign of a 0.0 in default rounding mode (which `fadd` implies) is the bitwise and of the sign bits of the inputs, and consequently, it's associative in that case.

https://github.com/llvm/llvm-project/pull/93698