cmd/gc: fix ARM build

CL 2520 omitted to set the type for an OCONVNOP node.
Typechecking obviously cannot do it for us.

5g inserts float64 <--> [u]int64 conversions at walk time.
The missing type caused it to crash.

Change-Id: Idce381f219bfef2e3a3be38d3ba3c258b71310ae
Reviewed-on: https://go-review.googlesource.com/2640Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/gc/range.c

@@ -185,6 +185,7 @@ walkrange(Node *n)
 				tmp->bounded = 1;
 				tmp = nod(OADDR, tmp, N);
 				tmp = nod(OCONVNOP, tmp, N);
+				tmp->type = ptrto(types[TUINT8]);
 				n->nbody = list(n->nbody, nod(OAS, hp, tmp));
 
 				// hn = len(a) * sizeof(elem(a))