gc: allow select case expr = <-c

Fixes #1139.

R=ken2
CC=golang-dev
https://golang.org/cl/2194046

src/cmd/gc/go.y

@@ -461,7 +461,7 @@ case:
 		}
 		break;
 	}
-|	LCASE name '=' expr ':'
+|	LCASE expr '=' expr ':'
 	{
 		// will be converted to OCASE
 		// right will point to next case

test/chan/select4.go

@@ -0,0 +1,25 @@
+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+package main
+
+func f() *int {
+	println("BUG: called f")
+	return new(int)
+}
+
+func main() {
+	var x struct {
+		a int
+	}
+	c := make(chan int, 1)
+	c1 := make(chan int)
+	c <- 42
+	select {
+	case *f() = <-c1:
+		// nothing
+	case x.a = <-c:
+		if x.a != 42 {
+			println("BUG:", x.a)
+		}
+	}
+}