[DAGCombiner] Use generalized pattern match for visitFSUBForFMACombine.

The patch makes visitFSUBForFMACombine serve vp.fsub too. It helps DAGCombiner
to fuse vp.fsub and vp.fmul patterns to vp.fma.

Reviewed By: luke

Differential Revision: https://reviews.llvm.org/D149821