Implement @owned to @unowned retain value conversion.

If a value is returned as @owned, we can move the epilogue retain
to the caller and convert the return value to @unowned. This gives
ARC optimizer more freedom to optimize the retain out on the caller's
side.

It appears that epilgue retains are harder to find than epilogue
releases. Most of the time they are not in the return block.

(1) Sometimes, they are in predecessors
(2) Sometimes they come from a call which returns an @owned return value.
This should be improved if we fix (1) and go bottom up.
(3) We do not handle exploded retain_value.

Currently, this catches a small number of opportunities.

We probably need to improve epilogue retain matcher if we are to handle
more cases.

This is part of rdar://24022375.

We also need some refactoring in the pass. e.g. break functions into smaller
functions. I will do with subsequent commit.

include/swift/SIL/Mangle.h

@@ -139,6 +139,18 @@ class FunctionSignatureSpecializationMangler
 
   friend class SpecializationMangler<FunctionSignatureSpecializationMangler>;
 
+  using ReturnValueModifierIntBase = uint16_t;
+  enum class ReturnValueModifier : ReturnValueModifierIntBase {
+    // Option Space 4 bits (i.e. 16 options).
+    Unmodified=0,
+    First_Option=0, Last_Option=31,
+
+    // Option Set Space. 12 bits (i.e. 12 option).
+    Dead=32,
+    OwnedToUnowned=64,
+    First_OptionSetEntry=32, LastOptionSetEntry=32768,
+  };
+
   // We use this private typealias to make it easy to expand ArgumentModifier's
   // size if we need to.
   using ArgumentModifierIntBase = uint16_t;
@@ -162,6 +174,8 @@ class FunctionSignatureSpecializationMangler
                             NullablePtr<SILInstruction>>;
   llvm::SmallVector<ArgInfo, 8> Args;
 
+  ReturnValueModifierIntBase ReturnValue;
+
 public:
   FunctionSignatureSpecializationMangler(SpecializationPass Pass,
                                          Mangle::Mangler &M, SILFunction *F);
@@ -173,6 +187,7 @@ public:
   void setArgumentSROA(unsigned ArgNo);
   void setArgumentBoxToValue(unsigned ArgNo);
   void setArgumentBoxToStack(unsigned ArgNo);
+  void setReturnValueOwnedToUnowned();
 
 private:
   void mangleSpecialization();
@@ -181,6 +196,7 @@ private:
   void mangleClosureProp(ThinToThickFunctionInst *TTTFI);
   void mangleArgument(ArgumentModifierIntBase ArgMod,
                       NullablePtr<SILInstruction> Inst);
+  void mangleReturnValue(ReturnValueModifierIntBase RetMod);
 };
 
 } // end namespace swift