cmd/compile: keep variables alive in testing.B.Loop loops

For the loop body guarded by testing.B.Loop, we disable function inlining and devirtualization inside. The only legal form to be matched is `for b.Loop() {...}`.

For #61515

Change-Id: I2e226f08cb4614667cbded498a7821dffe3f72d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/612043
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Bypass: Junyang Shao <shaojunyang@google.com>
Commit-Queue: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

test/inline_testingbloop.go

@@ -0,0 +1,31 @@
+// errorcheck -0 -m=2
+
+// Copyright 2024 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.
+
+// Test no inlining of function calls in testing.B.Loop.
+// See issue #61515.
+
+package foo
+
+import "testing"
+
+func caninline(x int) int { // ERROR "can inline caninline"
+	return x
+}
+
+func cannotinline(b *testing.B) { // ERROR "b does not escape" "cannot inline cannotinline.*"
+	for i := 0; i < b.N; i++ {
+		caninline(1) // ERROR "inlining call to caninline"
+	}
+	for b.Loop() { // ERROR "skip inlining within testing.B.loop"
+		caninline(1)
+	}
+	for i := 0; i < b.N; i++ {
+		caninline(1) // ERROR "inlining call to caninline"
+	}
+	for b.Loop() { // ERROR "skip inlining within testing.B.loop"
+		caninline(1)
+	}
+}