[DAGCombiner] Tighten reasscociation of visitFMA

From LangRef, FMF contract should not enable reassociating to form
arbitrary contractions. So it should not help rearrange nodes like
(fma (fmul x, c1), c2, y) into (fma x, c1*c2, y).

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D89527