[SIL] Let alloc_box return a single value.

And use project_box to get to the address value.
SILGen now generates a project_box for each alloc_box.
And IRGen re-uses the address value from the alloc_box if the operand of project_box is an alloc_box.
This lets the generated code be the same as before.

Other than that most changes of this (quite large) commit are straightforward.

lib/SILOptimizer/LoopTransforms/COWArrayOpt.cpp

@@ -215,8 +215,7 @@ protected:
       }
 
       // An alloc_box returns its address as the second value.
-      assert((PI.Aggregate == V || PI.Aggregate == SILValue(V, 1)) &&
-             "Expected unary element addr inst.");
+      assert(PI.Aggregate && "Expected unary element addr inst.");
 
       // Recursively check for users after stripping this component from the
       // access path.