cmd/compile,runtime: remove ambiguously live logic

The previous CL introduced stack objects. This CL removes the old
ambiguously live liveness analysis. After this CL we're relying
on stack objects exclusively.

Update a bunch of liveness tests to reflect the new world.

Fixes #22350

Change-Id: I739b26e015882231011ce6bc1a7f426049e59f31
Reviewed-on: https://go-review.googlesource.com/c/134156
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

test/stackobj3.go

@@ -0,0 +1,93 @@
+// run
+
+// 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 test makes sure that ambiguously live arguments work correctly.
+
+package main
+
+import (
+	"runtime"
+)
+
+type HeapObj [8]int64
+
+type StkObj struct {
+	h *HeapObj
+}
+
+var n int
+var c int = -1
+
+func gc() {
+	// encourage heap object to be collected, and have its finalizer run.
+	runtime.GC()
+	runtime.GC()
+	runtime.GC()
+	n++
+}
+
+var null StkObj
+
+var sink *HeapObj
+
+//go:noinline
+func use(p *StkObj) {
+}
+
+//go:noinline
+func f(s StkObj, b bool) {
+	var p *StkObj
+	if b {
+		p = &s
+	} else {
+		p = &null
+	}
+	// use is required here to prevent the conditional
+	// code above from being executed after the first gc() call.
+	use(p)
+	// If b==false, h should be collected here.
+	gc() // 0
+	sink = p.h
+	gc() // 1
+	sink = nil
+	// If b==true, h should be collected here.
+	gc() // 2
+}
+
+func fTrue() {
+	var s StkObj
+	s.h = new(HeapObj)
+	c = -1
+	n = 0
+	runtime.SetFinalizer(s.h, func(h *HeapObj) {
+		// Remember at what phase the heap object was collected.
+		c = n
+	})
+	f(s, true)
+	if c != 2 {
+		panic("bad liveness")
+	}
+}
+
+func fFalse() {
+	var s StkObj
+	s.h = new(HeapObj)
+	c = -1
+	n = 0
+	runtime.SetFinalizer(s.h, func(h *HeapObj) {
+		// Remember at what phase the heap object was collected.
+		c = n
+	})
+	f(s, false)
+	if c != 0 {
+		panic("bad liveness")
+	}
+}
+
+func main() {
+	fTrue()
+	fFalse()
+}