[mips] Move the code which does dag-combine for multiply-add/sub nodes to

[mips] Move the code which does dag-combine for multiply-add/sub nodes to
derived class MipsSETargetLowering.

We shouldn't be generating madd/msub nodes if target is Mips16, since Mips16
doesn't have support for multipy-add/sub instructions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178404 91177308-0d34-0410-b5e6-96231b3b80d8