DAGCombiner: Continue combining if FoldConstantArithmetic() fails.

DAG.FoldConstantArithmetic() can fail even though both operands are
Constants if OpaqueConstants are involved. Continue trying other combine
possibilities in tis case.

Differential Revision: http://reviews.llvm.org/D6946

Somewhat related to PR21801 / rdar://19211454

llvm-svn: 237822