cmd/compile: don't move nilCheck operations during tighten

Nil checks need to stay in their original blocks. They cannot
be moved to a following conditionally-executed block.

Fixes #72860

Change-Id: Ic2d66cdf030357d91f8a716a004152ba4c016f77
Reviewed-on: https://go-review.googlesource.com/c/go/+/657715
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

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

@@ -43,6 +43,10 @@ func tighten(f *Func) {
 				// SelectN is typically, ultimately, a register.
 				continue
 			}
+			if opcodeTable[v.Op].nilCheck {
+				// Nil checks need to stay in their block. See issue 72860.
+				continue
+			}
 			// Count arguments which will need a register.
 			narg := 0
 			for _, a := range v.Args {

test/fixedbugs/issue72860.go

@@ -0,0 +1,24 @@
+// 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
+
+//go:noinline
+func f(p *int, b bool) int {
+	valid := *p >= 0
+	if !b || !valid {
+		return 5
+	}
+	return 6
+}
+func main() {
+	defer func() {
+		if e := recover(); e == nil {
+			println("should have panicked")
+		}
+	}()
+	f(nil, false)
+}