Refactor the multiply-accumulate combines to act on

ARMISD::ADD[CE] nodes, instead of the generic ISD::ADD[CE].

Summary:
This allows for some simplification because the combines
are no longer limited to just one go at the node before
it gets legalized into an ARM target-specific one.

Reviewers: jmolloy, rogfer01

Subscribers: aemerson, llvm-commits, rengolin

Differential Revision: https://reviews.llvm.org/D30401

llvm-svn: 297453