cmd/compile: avoid remainder in loopbce when increment=0

For non-unit increment, loopbce checks to see if the
increment evenly divides the difference between (constant)
loop start and end.  This test panics when the increment
is zero.

Fix: check for zero, if found, don't optimize the loop.

Also added missing copyright notice to loopbce.go.

Fixes #26043.

Change-Id: I5f460104879cacc94481949234c9ce8c519d6380
Reviewed-on: https://go-review.googlesource.com/120759
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -1,3 +1,7 @@
+// Copyright 2018 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.
+
 package ssa
 
 import "fmt"
@@ -154,7 +158,7 @@ nextb:
 		// if the increment is ±1 or when the limits are constants.
 		if inc.AuxInt != 1 && inc.AuxInt != -1 {
 			ok := false
-			if min.Op == OpConst64 && max.Op == OpConst64 {
+			if min.Op == OpConst64 && max.Op == OpConst64 && inc.AuxInt != 0 {
 				if max.AuxInt > min.AuxInt && max.AuxInt%inc.AuxInt == min.AuxInt%inc.AuxInt { // handle overflow
 					ok = true
 				}

test/fixedbugs/issue26043.go

@@ -0,0 +1,32 @@
+// compile
+
+// Copyright 2018 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.
+
+// This program results in a loop inferred to increment
+// j by 0, causing bounds check elimination to attempt
+// something%0, which panics (in the bug).
+
+package q
+
+func f() {
+	var s1 string
+	var b bool
+	if b {
+		b = !b
+		s1 += "a"
+	}
+
+	var s2 string
+	var i, j int
+	if (s1 <= "") || (s2 >= "") {
+		j = len(s1[:6])
+	} else {
+		i = len("b")
+	}
+
+	for j < 0 {
+		j += i
+	}
+}