cmd/gc: shorten name used for map bucket type

Before:
type.struct { buckets *struct { overflow *struct { overflow *struct { overflow *struct { overflow *struct { overflow *<...>; keys [8]string; values [8]*"".RangeTable }; keys [8]string; values [8]*"".RangeTable }; keys [8]string; values [8]*"".RangeTable }; keys [8]string; values [8]*"".RangeTable }; keys [8]string; values [8]*"".RangeTable }; oldbuckets *struct { overflow *struct { overflow *struct { overflow *struct { overflow *struct { overflow *<...>; keys [8]string; values [8]*"".RangeTable }; keys [8]string; values [8]*"".RangeTable }; keys [8]string; values [8]*"".RangeTable }; keys [8]string; values [8]*"".RangeTable }; keys [8]string; values [8]*"".RangeTable } }

After:
type.map.bucket[string]*"".RangeTable

This makes debugging maps a little nicer, and it takes up less space in the binary.

R=golang-dev, r
CC=golang-dev, khr
https://golang.org/cl/15110044

src/cmd/gc/fmt.c

@@ -700,6 +700,13 @@ typefmt(Fmt *fp, Type *t)
 		return 0;
 
 	case TSTRUCT:
+		// Format the bucket struct for map[x]y as map.bucket[x]y.
+		// This avoids a recursive print that generates very long names.
+		if(t->hmap != T) {
+			t = t->hmap;
+			return fmtprint(fp, "map.bucket[%T]%T", t->down, t->type);
+		}
+
 		if(t->funarg) {
 			fmtstrcpy(fp, "(");
 			if(fmtmode == FTypeId || fmtmode == FErr) {	// no argument names on function signature, and no "noescape" tags

src/cmd/gc/reflect.c

@@ -229,6 +229,7 @@ hmap(Type *t)
 	h->width = offset;
 	h->local = t->local;
 	t->hmap = h;
+	h->hmap = t;
 	return h;
 }