[ARM] Fold (fadd x, (vselect c, y, -1.0)) into (vselect c, (fadd x, y), x)

This is similar to D113574, but as a DAG combine, not tablegen patterns.
Doing the fold as a DAG combine allows the fadd to be folded with a
fmul, finally producing a predicated vfma. It performs the same fold of
fadd(x, vselect(p, y, -0.0)) to vselect p, (fadd x, y), x) using -0.0 as
the identity value of a fadd.

Differential Revision: https://reviews.llvm.org/D113584