[dev.ssa] cmd/compile: make sure entry block has no predecessors

Fix one test that build a violating CFG.

Change-Id: Ie0296ced602984d914a70461c76559c507ce2510
Reviewed-on: https://go-review.googlesource.com/13621
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/cmd/compile/internal/ssa/check.go

@@ -157,6 +157,10 @@ func checkFunc(f *Func) {
 		}
 	}
 
+	if len(f.Entry.Preds) > 0 {
+		f.Fatalf("entry block %s of %s has predecessor(s) %v", f.Entry, f.Name, f.Entry.Preds)
+	}
+
 	// Check to make sure all Values referenced are in the function.
 	for _, b := range f.Blocks {
 		for _, v := range b.Values {

src/cmd/compile/internal/ssa/dom_test.go

@@ -317,11 +317,13 @@ func TestDominatorsMultPredRev(t *testing.T) {
 	c := testConfig(t)
 	fun := Fun(c, "entry",
 		Bloc("entry",
+			Goto("first")),
+		Bloc("first",
 			Valu("mem", OpArg, TypeMem, 0, ".mem"),
 			Valu("p", OpConstBool, TypeBool, 0, true),
 			Goto("a")),
 		Bloc("a",
-			If("p", "b", "entry")),
+			If("p", "b", "first")),
 		Bloc("b",
 			Goto("c")),
 		Bloc("c",
@@ -330,10 +332,11 @@ func TestDominatorsMultPredRev(t *testing.T) {
 			Exit("mem")))
 
 	doms := map[string]string{
-		"a":    "entry",
-		"b":    "a",
-		"c":    "b",
-		"exit": "c",
+		"first": "entry",
+		"a":     "first",
+		"b":     "a",
+		"c":     "b",
+		"exit":  "c",
 	}
 
 	CheckFunc(fun.f)