test: add escape regress tests for runtime and sync atomics

There weren't any tests to make sure these work correctly, and this
led to escape analysis regressions in both linux/s390x and js/wasm.

The underlying issue that cmd/compile is only getting some of these
correct because escape analysis doesn't understand //go:linkname is
still present, but at least this addresses the fragility aspect.

Updates #15283.

Change-Id: I546aee1899d098b2e3de45e9b33c3ca22de485f8
Reviewed-on: https://go-review.googlesource.com/c/go/+/172420
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

test/escape_runtime_atomic.go

+// errorcheck -0 -m -l
+
+// Copyright 2019 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 escape analysis for runtime/internal/atomic.
+
+package escape
+
+import (
+	"runtime/internal/atomic"
+	"unsafe"
+)
+
+// BAD: should be "leaking param content".
+func Loadp(addr unsafe.Pointer) unsafe.Pointer { // ERROR "leaking param: addr"
+	return atomic.Loadp(addr)
+}
+
+var ptr unsafe.Pointer
+
+func Storep() {
+	var x int // ERROR "moved to heap: x"
+	atomic.StorepNoWB(unsafe.Pointer(&ptr), unsafe.Pointer(&x))
+}
+
+func Casp1() {
+	// BAD: x doesn't need to be heap allocated
+	var x int // ERROR "moved to heap: x"
+	var y int // ERROR "moved to heap: y"
+	atomic.Casp1(&ptr, unsafe.Pointer(&x), unsafe.Pointer(&y))
+}

test/escape_sync_atomic.go

+// errorcheck -0 -m -l
+
+// Copyright 2019 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 escape analysis for sync/atomic.
+
+package escape
+
+import (
+	"sync/atomic"
+	"unsafe"
+)
+
+// BAD: should be "leaking param content".
+func LoadPointer(addr *unsafe.Pointer) unsafe.Pointer { // ERROR "leaking param: addr"
+	return atomic.LoadPointer(addr)
+}
+
+var ptr unsafe.Pointer
+
+func StorePointer() {
+	var x int // ERROR "moved to heap: x"
+	atomic.StorePointer(&ptr, unsafe.Pointer(&x))
+}
+
+func SwapPointer() {
+	var x int // ERROR "moved to heap: x"
+	atomic.SwapPointer(&ptr, unsafe.Pointer(&x))
+}
+
+func CompareAndSwapPointer() {
+	// BAD: x doesn't need to be heap allocated
+	var x int // ERROR "moved to heap: x"
+	var y int // ERROR "moved to heap: y"
+	atomic.CompareAndSwapPointer(&ptr, unsafe.Pointer(&x), unsafe.Pointer(&y))
+}