Add a new target independent COPY instruction and code to lower it.

The COPY instruction is intended to replace the target specific copy
instructions for virtual registers as well as the EXTRACT_SUBREG and
INSERT_SUBREG instructions in MachineFunctions. It won't we used in a selection
DAG.

COPY is lowered to native register copies by LowerSubregs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@107529 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Target/TargetOpcodes.h

@@ -75,7 +75,11 @@ namespace TargetOpcode {
     /// e.g. v1027 = REG_SEQUENCE v1024, 3, v1025, 4, v1026, 5
     /// After register coalescing references of v1024 should be replace with
     /// v1027:3, v1025 with v1027:4, etc.
-    REG_SEQUENCE = 12
+    REG_SEQUENCE = 12,
+
+    /// COPY - Target-independent register copy. This instruction can also be
+    /// used to copy between subregisters of virtual registers.
+    COPY = 13
   };
 } // end namespace TargetOpcode
 } // end namespace llvm