cmd/compile: handle infinite loops in shortcircuit pass

The newly upgraded shortcircuit pass attempted to remove infinite loops.
Stop doing that.

Fixes #33903

Change-Id: I0fc9c1b5f2427e54ce650806602ef5e3ad65aca5
Reviewed-on: https://go-review.googlesource.com/c/go/+/192144
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -50,7 +50,7 @@ func shortcircuit(f *Func) {
 		}
 	}
 
-	// Step 3: Redirect control flow around known branches.
+	// Step 2: Redirect control flow around known branches.
 	// p:
 	//   ... goto b ...
 	// b: <- p ...
@@ -124,7 +124,6 @@ func shortcircuitBlock(b *Block) bool {
 		if a.Op != OpConstBool {
 			continue
 		}
-		changed = true
 		// The predecessor we come in from.
 		e1 := b.Preds[i]
 		p := e1.b
@@ -138,8 +137,15 @@ func shortcircuitBlock(b *Block) bool {
 		}
 		e2 := b.Succs[si]
 		t := e2.b
+		if p == b || t == b {
+			// This is an infinite loop; we can't remove it. See issue 33903.
+			continue
+		}
 		ti := e2.i
 
+		// Update CFG and Phis.
+		changed = true
+
 		// Remove b's incoming edge from p.
 		b.removePred(i)
 		n := len(b.Preds)

test/fixedbugs/issue33903.go

+// compile
+
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Check that the shortcircuit pass correctly handles infinite loops.
+
+package p
+
+func f() {
+	var p, q bool
+	for {
+		p = p && q
+	}
+}