[AutoDiff] Revisit differentiation of struct instruction with aggregate adjoint value. (swiftlang#25234)

In the latest Swift, no calls to an initializer will emit a `struct` instruction, so differentiation of the `struct` instruction will only be for differentiating struct initializer functions themselves. In struct initializer functions, since there's only one `struct` operation, `AdjointEmitter::visitStructInst` will never get an adjoint value of kind `AdjointValueKind::Aggregate`. This PR marks that case unreachable.

Author: rxwei

lib/SILOptimizer/Mandatory/Differentiation.cpp

@@ -4541,8 +4541,11 @@ class AdjointEmitter final : public SILInstructionVisitor<AdjointEmitter> {
       break;
     }
     case AdjointValueKind::Aggregate: {
-      llvm_unreachable("Unhandled. Are you trying to differentiate a "
-                       "memberwise initializer?");
+      // Note: All user-called initializations go through the calls to the
+      // initializer, and synthesized initializers only have one level of struct
+      // formation which will not result into any aggregate adjoint valeus.
+      llvm_unreachable("Aggregate adjoint values should not occur for `struct` "
+                       "instructions");
     }
     }
   }

test/AutoDiff/simple_math.swift

@@ -184,7 +184,8 @@ SimpleMathTests.test("StructMemberwiseInitializer") {
 
   let 𝛁foo = pullback(at: Float(4), in: { input -> Foo in
     let foo = Foo(stored: input)
-    return foo + foo
+    let foo2 = foo + foo
+    return Foo(stored: foo2.stored)
   })(Foo.TangentVector(stored: 1))
   expectEqual(2, 𝛁foo)