[DAGCombiner, PowerPC] allow X - (fpext(-Y) --> X + fpext(Y) with multiple uses

This is a transform that I limited in instcombine in rL329821 because it was 
creating more instructions in IR when the cast has multiple uses.

But if the cast is free, then we can do the transform regardless of other
uses because it improves the potential throughput of the calculation by
removing a dependency on the fneg.

Differential Revision: https://reviews.llvm.org/D45598

llvm-svn: 330098