Subzero: Allow builds with assertions disabled.

Subzero: Allow builds with assertions disabled.

1. Setting command-line make variable NOASSERT=1 adds -DNDEBUG and builds in a separate directory.  By default, we still get Release+Asserts.

2. Add "(void)foo;" as necessary when foo is only used in an assert(), to remove warnings.

3. Minimize inclusion of llvm/Support/Timer.h because it adds warnings.

4. Call validateLiveness() only when asserts are enabled, because it's relatively expensive.

BUG= none
R=jvoung@chromium.org

Committed: https://gerrit.chromium.org/gerrit/gitweb?p=native_client/pnacl-subzero.git;a=commit;h=9c234e2a5ab681825bb92de3c7f3c74b38cd2f8a

[Jim Stichnoth, 2014-10-01 14:50:39]

stichnot@chromium.org changed reviewers:
+ jvoung@chromium.org

[jvoung (off chromium), 2014-10-01 15:30:07]

lgtm

[jvoung (off chromium), 2014-10-01 15:38:29]

LLVM was also changed to disable stat counters when asserts are disabled, but
there it is more prolific and heavy weight -- it uses atomic operations and a
memory fence for every operation increment operation for lazy registration.

[Jim Stichnoth, 2014-10-01 16:20:46]

On 2014/10/01 15:38:29, jvoung wrote:
> LLVM was also changed to disable stat counters when asserts are disabled, but
> there it is more prolific and heavy weight -- it uses atomic operations and a
> memory fence for every operation increment operation for lazy registration.

That makes sense and I'll keep it in mind going forward.  For now, I want to
keep pass timing enabled even without asserts, for performance analysis.

BTW, when we add stats counters, my inclination is to make them non-atomic and
potentially racy, unless we really need total accuracy.

[Jim Stichnoth, 2014-10-01 16:28:52]

Committed patchset #2 (id:20001) manually as
9c234e2a5ab681825bb92de3c7f3c74b38cd2f8a (presubmit successful).