[DAGCombiner] Improve FMA support for interpolation patterns

This patch adds support for combining patterns such as (FMUL(FADD(1.0, x), y)) and (FMUL(FSUB(x, 1.0), y)) to their FMA equivalents.

This is useful in particular for linear interpolation cases such as (FADD(FMUL(x, t), FMUL(y, FSUB(1.0, t))))

Differential Revision: http://reviews.llvm.org/D13003

llvm-svn: 248210