SelectionDAG: Prefer to combine multiplication with less uses for fma

Summary:
For example:

  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

Reviewers: hfinkel, apazos, jingyue, ohsallen, arsenm

Subscribers: arsenm, jholewinski, llvm-commits

Differential Revision: http://reviews.llvm.org/D11855

llvm-svn: 244649