Adding support for multiple returns in compiled functions.

src/compiler/linkage.h

Index: src/compiler/linkage.h
diff --git a/src/compiler/linkage.h b/src/compiler/linkage.h
index b25fe413c96c40f69fa5a1e942bb3b2f9aa08644..2e223c9e25061d147738d81ecba9c698a3630f92 100644
--- a/src/compiler/linkage.h
+++ b/src/compiler/linkage.h
@@ -304,12 +304,14 @@ class Linkage : public ZoneObject {
   }
 
   // Get the location where this function should place its return value.
-  LinkageLocation GetReturnLocation() const {
-    return incoming_->GetReturnLocation(0);
+  LinkageLocation GetReturnLocation(size_t index = 0) const {
+    return incoming_->GetReturnLocation(index);
   }
 
   // Get the machine type of this function's return value.
-  MachineType GetReturnType() const { return incoming_->GetReturnType(0); }
+  MachineType GetReturnType(size_t index = 0) const {
+    return incoming_->GetReturnType(index);
+  }
 
   // Get the frame offset for a given spill slot. The location depends on the
   // calling convention and the specific frame layout, and may thus be