[test] Protect against infinite loops in LiveRange logic.

[test] Protect against infinite loops in LiveRange logic.

A subset of the LiveRangeUnitTests (SplitInvalidPreStart,
InvalidSplitEnd, SplitInvalidPostEnd) fail or hang on AIX in release
mode.

These tests fork a child which is expected to crash in
register-allocator code after feeding in bad inputs.

In debug mode, they behave as expected due to hitting a debug assert.

In release mode, however, the tests rely only on the fact that
dereferencing a null pointer will cause a SEGFAULT.  This is true on
most platforms, but not AIX.  An AIX process has valid low memory
pages mapped for reading and will not fault.  Thus, these tests fail
or hang because the child process survives the load from address zero
and either completes (with undefined results) or goes into an infinite
loop.

R=bmeurer@chromium.org, danno@chromium.org, michael_dawson@ca.ibm.com
BUG=

Committed: https://crrev.com/4ddc9f1cc82f739f6da657e4076c291cbcdab765
Cr-Commit-Position: refs/heads/master@{#31090}

[MTBrandyberry, 2015-10-02 16:25:03]

The CQ bit was checked by mbrandy@us.ibm.com to run a CQ dry run

[commit-bot: I haz the power, 2015-10-02 16:25:16]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1384733002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1384733002/1

[commit-bot: I haz the power, 2015-10-02 16:48:25]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-10-02 16:48:26]

Dry run: This issue passed the CQ dry run.

[Benedikt Meurer, 2015-10-02 16:54:52]

I'd rather disable the test in release mode and leave the DCHECK as is, see
bits-unittest.cc for an example how to do that.

[MTBrandyberry, 2015-10-02 17:33:43]

On 2015/10/02 16:54:52, Benedikt Meurer wrote:
> I'd rather disable the test in release mode and leave the DCHECK as is, see
> bits-unittest.cc for an example how to do that.

Thanks.  Uploaded patch set 2.

[Benedikt Meurer, 2015-10-02 18:13:28]

Nice, thanks. LGTM

[MTBrandyberry, 2015-10-02 18:20:49]

The CQ bit was checked by mbrandy@us.ibm.com

[commit-bot: I haz the power, 2015-10-02 18:21:04]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1384733002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1384733002/20001

[commit-bot: I haz the power, 2015-10-02 18:44:58]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-10-02 18:45:08]

Patchset 2 (id:??) landed as
https://crrev.com/4ddc9f1cc82f739f6da657e4076c291cbcdab765
Cr-Commit-Position: refs/heads/master@{#31090}