[SIL CCP] Add the early constant propagation pass skeleton.

The pass folds a single operation: int_sadd_with_overflow and issues an
error on detectable overflow.

Swift SVN r7204

include/swift/SIL/SILValue.h

@@ -25,6 +25,7 @@
 namespace swift {
   class SILTypeList;
   class Operand;
+  class SILValue;
   class ValueBaseUseIterator;
   class ValueUseIterator;
 
@@ -73,6 +74,7 @@ public:
   inline ValueBaseUseIterator use_begin();
   inline ValueBaseUseIterator use_end();
   inline Range<ValueBaseUseIterator> getUses();
+  void replaceAllUsesWith(SILValue V);
 
   /// Pretty-print the value.
   void dump() const;