Fix referencing passed-by-ref structs in win abi

The double-dereference necessary to go from the function argument to the
actual location (in the case of a large structure pass) was incorrectly
happening in the field codegen. This worked if the structure was only
ever referenced as `obj.xyz`, but if `obj` is directly used (for
example, but assignment) then the address wasn't correctly calculated.

src/codegen.in.c

@@ -219,6 +219,11 @@ static void gen_addr(Node* node) {
       // Local variable
       if (node->var->is_local) {
         ///| lea rax, [rbp+node->var->offset]
+#if X64WIN
+        if (node->var->is_param_passed_by_reference) {
+          ///| mov rax, [rax]
+        }
+#endif
         return;
       }
 
@@ -272,11 +277,6 @@ static void gen_addr(Node* node) {
       return;
     case ND_MEMBER:
       gen_addr(node->lhs);
-#if X64WIN
-      if (node->lhs->kind == ND_VAR && node->lhs->var->is_param_passed_by_reference) {
-        ///| mov rax, [rax]
-      }
-#endif
       ///| add rax, node->member->offset
       return;
     case ND_FUNCALL:
@@ -2090,7 +2090,7 @@ static void assign_lvar_offsets(Obj* prog) {
             // such and then either assign a register or stack slot for the
             // reference.
             // outaf("by ref %s\n", var->name);
-            // var->passed_by_reference = true;
+            assert(var->is_param_passed_by_reference);
           }
 
           // If the pointer to a referenced value or the value itself can be

test/struct_copy.c

@@ -0,0 +1,27 @@
+#include <stdio.h>
+static void printint(int x) {
+  printf("%d\n", x);
+}
+
+struct B {
+  int b;
+  int c;
+  int x;
+};
+
+struct A {
+  struct B b;
+};
+
+void A(struct B b) {
+  struct A a;
+  a.b = b;
+  printint(a.b.b);
+  printint(a.b.c);
+  printint(a.b.x);
+}
+
+int main(void) {
+  struct B b = (struct B){3, 2, 1};
+  A(b);
+}