Add an assert to detect an instruction being added as an operand of itself.

Swift SVN r16303

include/swift/SIL/SILValue.h

@@ -280,6 +280,9 @@ public:
     // It's probably not worth optimizing for the case of switching
     // operands on a single value.
     removeFromCurrent();
+    assert(cast<ValueBase>(Owner) != newValue.getDef()
+           && "Cannot add a value as an operand of the instruction that defines"
+              " it!");
     TheValue = newValue;
     insertIntoCurrent();
   }