cmd/compile: add typMap

Also, add two uses of Key and Val that I missed earlier.
As before, direct writes to Down and Type remain in bimport.

Change-Id: I487aa975926b30092db1ad74ace17994697117c1
Reviewed-on: https://go-review.googlesource.com/21330Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/export.go

@@ -301,8 +301,8 @@ func dumpexporttype(t *Type) {
 		dumpexporttype(t.Results())
 		dumpexporttype(t.Params())
 	case TMAP:
-		dumpexporttype(t.Type)
-		dumpexporttype(t.Down) // key
+		dumpexporttype(t.Val())
+		dumpexporttype(t.Key())
 	case TARRAY, TCHAN, TPTR32, TPTR64:
 		dumpexporttype(t.Type)
 	}

src/cmd/compile/internal/gc/subr.go

@@ -382,27 +382,24 @@ func maptype(key *Type, val *Type) *Type {
 				Yyerror("invalid map key type %v", key)
 			}
 
-			// will be resolved later.
 		case TANY:
+			// will be resolved later.
 			break
 
-			// map[key] used during definition of key.
-		// postpone check until key is fully defined.
-		// if there are multiple uses of map[key]
-		// before key is fully defined, the error
-		// will only be printed for the first one.
-		// good enough.
 		case TFORW:
+			// map[key] used during definition of key.
+			// postpone check until key is fully defined.
+			// if there are multiple uses of map[key]
+			// before key is fully defined, the error
+			// will only be printed for the first one.
+			// good enough.
 			if key.Maplineno == 0 {
 				key.Maplineno = lineno
 			}
 		}
 	}
 
-	t := typ(TMAP)
-	t.Down = key
-	t.Type = val
-	return t
+	return typMap(key, val)
 }
 
 // methcmp sorts by symbol, then by package path for unexported symbols.

src/cmd/compile/internal/gc/type.go

@@ -267,6 +267,14 @@ func typeChan(elem *Type, dir uint8) *Type {
 	return t
 }
 
+// typMap returns a new map Type with key type k and element (aka value) type v.
+func typMap(k, v *Type) *Type {
+	t := typ(TMAP)
+	t.Down = k
+	t.Type = v
+	return t
+}
+
 // typPtr returns a new pointer type pointing to t.
 func typPtr(elem *Type) *Type {
 	t := typ(Tptr)