[InstCombine] Use SimplifyFMulInst to simplify multiply in fma.

This allows us to fold fma's that multiply with 0.0. Also, the
multiply by 1.0 case is handled there as well. The fneg/fabs cases
are not handled by SimplifyFMulInst, so we need to keep them.

Reviewers: spatel, anemet, lebedev.ri

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D67351

llvm-svn: 371518