[llvm] [InstCombine] Canonicalize reassoc contract fmuladd to fmul + fadd (PR #90434)

[via llvm-commits]

vfdff wrote:

   Hi @andykaylor, sorry for the late reply. I generate the origin IR from **classic flang** is some thing like https://gcc.godbolt.org/z/vdhnEq9rY (with a **fast** flag).
 > %add5 = call fast double @llvm.fmuladd.f64 (double %0, double %1, double %add)

   As I think the **fast** flag is too strict for the current conversion（fmulall -> fmul + fadd）,  so I change the **fast** to **reassoc contract**, and that is the input IR dectription in this topic (PR90379).  

https://github.com/llvm/llvm-project/pull/90434