cmd/compile: disable flaky test

The test is inherently racy and vulnerable to starvation,
and within all.bash on some platforms that means it flakes.
Test is kept because it can be useful standalone to verify
behavior of GOEXPERIMENT=preeemptibleloops, and there is
likely to be further development of this feature in the
future.

There's also some question as to why it is flaking, because
though technically this is permitted, it's very odd in this
simple case.

Fixes golang#18589.

Change-Id: Ia0dd9037285c4a03122da4012c96981c9cc43b60
Reviewed-on: https://go-review.googlesource.com/35051
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

test/fixedbugs/issue10958.go

@@ -1,12 +1,21 @@
-// +build !nacl
-// buildrun -t 2  -gcflags=-d=ssa/insert_resched_checks/on,ssa/check/on
+// +build !nacl,disabled
+// buildrun -t 10  -gcflags=-d=ssa/insert_resched_checks/on,ssa/check/on
 
 // Copyright 2016 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 is disabled because it flakes when run in all.bash
+// on some platforms, but is useful standalone to verify
+// that rescheduling checks are working (and we may wish
+// to investigate the flake, since it suggests that the
+// loop rescheduling check may not work right on those
+// platforms).
+
 // This checks to see that call-free infinite loops do not
-// block garbage collection.
+// block garbage collection.  IF YOU RUN IT STANDALONE without
+// -gcflags=-d=ssa/insert_resched_checks/on in a not-experimental
+// build, it should hang.
 
 package main