cmd/compile: don't rely on loop info when there are irreducible loops

Loop information is sketchy when there are irreducible loops.
Sometimes blocks inside 2 loops can be recorded as only being part of
the outer loop. That causes tighten to move values that want to move
into such a block to move out of the loop altogether, breaking the
invariant that operations have to be scheduled after their args.

Fixes #75569

Change-Id: Idd80e6d2268094b8ae6387563081fdc1e211856a
Reviewed-on: https://go-review.googlesource.com/c/go/+/706355
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>

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

@@ -123,6 +123,8 @@ func tighten(f *Func) {
 
 		// If the target location is inside a loop,
 		// move the target location up to just before the loop head.
+		if !loops.hasIrreducible {
+			// Loop info might not be correct for irreducible loops. See issue 75569.
 			for _, b := range f.Blocks {
 				origloop := loops.b2l[b.ID]
 				for _, v := range b.Values {
@@ -138,6 +140,7 @@ func tighten(f *Func) {
 					}
 				}
 			}
+		}
 
 		// Move values to target locations.
 		for _, b := range f.Blocks {

test/fixedbugs/issue75569.go

@@ -0,0 +1,77 @@
+// run
+
+// Copyright 2025 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 main
+
+func fff(a []int, b bool, p, q *int) {
+outer:
+	n := a[0]
+	a = a[1:]
+	switch n {
+	case 1:
+		goto one
+	case 2:
+		goto two
+	case 3:
+		goto three
+	case 4:
+		goto four
+	}
+
+one:
+	goto inner
+two:
+	goto outer
+three:
+	goto inner
+four:
+	goto innerSideEntry
+
+inner:
+	n = a[0]
+	a = a[1:]
+	switch n {
+	case 1:
+		goto outer
+	case 2:
+		goto inner
+	case 3:
+		goto innerSideEntry
+	default:
+		return
+	}
+innerSideEntry:
+	n = a[0]
+	a = a[1:]
+	switch n {
+	case 1:
+		goto outer
+	case 2:
+		goto inner
+	case 3:
+		goto inner
+	}
+	ggg(p, q)
+	goto inner
+}
+
+var b bool
+
+func ggg(p, q *int) {
+	n := *p + 5 // this +5 ends up in the entry block, well before the *p load
+	if b {
+		*q = 0
+	}
+	*p = n
+}
+
+func main() {
+	var x, y int
+	fff([]int{4, 4, 4}, false, &x, &y)
+	if x != 5 {
+		panic(x)
+	}
+}