Add OPC_CheckChildSame0-3 to the DAG isel matcher. This replaces sequences of MoveChild, CheckSame, MoveParent. Saves 846 bytes from the X86 DAG isel matcher, ~300 from ARM, ~840 from Hexagon.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191940 91177308-0d34-0410-b5e6-96231b3b80d8