[mips] Fix definitions of multiply, multiply-add/sub and divide instructions.

[mips] Fix definitions of multiply, multiply-add/sub and divide instructions.

The new instructions have explicit register output operands and use table-gen
patterns instead of C++ code to do instruction selection.

Mips16's instructions are unaffected by this change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178403 91177308-0d34-0410-b5e6-96231b3b80d8