go.tools/pointer: fix constraint generation bug in *ssa.Select.

The previous code assumed that each channel item contained one field.

+ regression test.

R=crawshaw
CC=golang-dev
https://golang.org/cl/25480044

pointer/gen.go

@@ -994,7 +994,7 @@ func (a *analysis) genInstr(cgn *cgnode, instr ssa.Instruction) {
 			switch st.Dir {
 			case ast.RECV:
 				a.genLoad(cgn, recv, st.Chan, 0, elemSize)
-				recv++
+				recv += nodeid(elemSize)
 
 			case ast.SEND:
 				a.genStore(cgn, st.Chan, a.valueNode(st.Send), 0, elemSize)

pointer/testdata/channels.go

@@ -90,9 +90,29 @@ func chan4() {
 	}
 }
 
+// Multi-word channel value in select with multiple receive cases.
+// (Regtest for a crash.)
+func chan5() {
+	type T struct {
+		x *int
+		y interface{}
+	}
+	ch := make(chan T)
+	ch <- T{new(int), incr} // @line ch5new
+	select {
+	case a := <-ch:
+		print(a.x) // @pointsto new@ch5new:13
+		print(a.y) // @types func(x int) int
+	case b := <-ch:
+		print(b.x) // @pointsto new@ch5new:13
+		print(b.y) // @types func(x int) int
+	}
+}
+
 func main() {
 	chan1()
 	chan2()
 	chan3()
 	chan4()
+	chan5()
 }