Refactor the constant folding code into it's own function. And call it from both

Refactor the constant folding code into it's own function. And call it from both
the SelectionDAG and DAGCombiner code. The only functionality change is that now
the DAG combiner is performing the constant folding for these operations instead
of being a no-op.

This is *not* in response to a bug, so there isn't a testcase.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@56550 91177308-0d34-0410-b5e6-96231b3b80d8