s6 = s0*s5; s2 = s6*s6 + s6; ... s4 = s6*s3;
We notice that it is possible for s2 is folded to fma (s0, s5, fmul (s6 s6)).
This only happens when Aggressive is true, otherwise hasOneUse() check
already prevents from folding the multiplication with more uses.
Test Plan: test/CodeGen/NVPTX/fma-assoc.ll
Patch by Xuetian Weng