test: fix escape_runtime_atomic.go

Casp1 is implemented in Go on js/wasm, so escape analysis correctly
determines that the "old" parameter does not escape (which is good).

Unfortunately, test/run.go doesn't have a way to indicate that ERROR
messages are optional, and cmd/compile only emits diagnostics for "var
x int" when it's moved to the heap; not when it stays on the stack.

To accomodate that this test currently passes on some GOARCHes but not
others, rewrite the Casp1 test to use "x := new(int)" and allow both
"new(int) escapes to heap" or "new(int) does not escape".

Updates #31525.

Change-Id: I40150a7ff9042f184386ccdb2d4d428f63e8ba4f
Reviewed-on: https://go-review.googlesource.com/c/go/+/172602
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/escape_runtime_atomic.go

@@ -26,8 +26,8 @@ func Storep() {
 }
 
 func Casp1() {
-	// BAD: x doesn't need to be heap allocated
-	var x int // ERROR "moved to heap: x"
+	// BAD: should always be "does not escape"
+	x := new(int) // ERROR "escapes to heap|does not escape"
 	var y int     // ERROR "moved to heap: y"
-	atomic.Casp1(&ptr, unsafe.Pointer(&x), unsafe.Pointer(&y))
+	atomic.Casp1(&ptr, unsafe.Pointer(x), unsafe.Pointer(&y))
 }