cmd/gc: add write barrier in copy of function parameters to heap

Found with GODEBUG=wbshadow=2 mode.
Eventually that will run automatically, but right now
it still detects other missing write barriers.

Change-Id: I1320d5340a9e421c779f24f3b170e33974e56e4f
Reviewed-on: https://go-review.googlesource.com/2278Reviewed-by: Rick Hudson <rlh@golang.org>

src/cmd/gc/typecheck.c

@@ -2786,6 +2786,7 @@ islvalue(Node *n)
 	case OIND:
 	case ODOTPTR:
 	case OCLOSUREVAR:
+	case OPARAM:
 		return 1;
 	case ODOT:
 		return islvalue(n->left);

src/cmd/gc/walk.c

@@ -462,6 +462,7 @@ walkexpr(Node **np, NodeList **init)
 	case ONONAME:
 	case OINDREG:
 	case OEMPTY:
+	case OPARAM:
 		goto ret;
 
 	case ONOT:
@@ -2519,7 +2520,7 @@ paramstoheap(Type **argin, int out)
 {
 	Type *t;
 	Iter savet;
-	Node *v;
+	Node *v, *as;
 	NodeList *nn;
 
 	nn = nil;
@@ -2544,8 +2545,13 @@ paramstoheap(Type **argin, int out)
 		if(v->alloc == nil)
 			v->alloc = callnew(v->type);
 		nn = list(nn, nod(OAS, v->heapaddr, v->alloc));
-		if((v->class & ~PHEAP) != PPARAMOUT)
-			nn = list(nn, nod(OAS, v, v->stackparam));
+		if((v->class & ~PHEAP) != PPARAMOUT) {
+			as = nod(OAS, v, v->stackparam);
+			v->stackparam->typecheck = 1;
+			typecheck(&as, Etop);
+			as = applywritebarrier(as, &nn);
+			nn = list(nn, as);
+		}
 	}
 	return nn;
 }