[DAGCombine] Fold the (fma -x, y, -z) to -(fma x, y, z)
This is a positive combination as long as the NEG is NOT free,
as we are reducing the number of NEG from two to one.
Differential Revision: https://reviews.llvm.org/D72312
steven.zhang on Jan 8 2020, 7:41 PM.Authored by