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/stackobj.go

@@ -0,0 +1,57 @@
+// 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.
+
+package main
+
+import (
+	"fmt"
+	"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++
+}
+
+func main() {
+	f()
+	gc() // prior to stack objects, heap object is not collected until here
+	if c < 0 {
+		panic("heap object never collected")
+	}
+	if c != 1 {
+		panic(fmt.Sprintf("expected collection at phase 1, got phase %d", c))
+	}
+}
+
+func f() {
+	var s StkObj
+	s.h = new(HeapObj)
+	runtime.SetFinalizer(s.h, func(h *HeapObj) {
+		// Remember at what phase the heap object was collected.
+		c = n
+	})
+	g(&s)
+	gc()
+}
+
+func g(s *StkObj) {
+	gc() // heap object is still live here
+	runtime.KeepAlive(s)
+	gc() // heap object should be collected here
+}