[lit] don't skip test times when executions are instantaneous

[ashay-github]

Summary

If a test executes quickly or if the timing resolution is too coarse,
the running time associated with a test could be zero. This happens
often for the //llvm/utils/lit/tests/reorder.py test on Windows, which
causes the test to fail non-deterministically.

This patch modifies the existing check, so that instead of skipping
zeros, lit now skips if the timing is None (presumably due to an error
in measurement). Recurring executions of the reorder.py test on Windows
seem to indicate that the test no longer exhibits occasional failure.

[stella.stamenova]

Thanks for fixing this long-standing bug!

[ashay-github]

Rebased with main branch.

[ashay-github]

Rebased with main branch.

llvm/utils/lit/lit/TestTimes.py

if os.path.exists(test_times_file):
                 test_times[path.strip('\n')] = float(time)
     return test_times
 
 
 def record_test_times(tests, lit_config):
     times_by_suite = {}
     for t in tests:
         assert t.suite.test_times is None
-        if not t.result.elapsed:
+        if t.result.elapsed is None:
             continue
         if not t.suite.exec_root in times_by_suite:
             times_by_suite[t.suite.exec_root] = read_test_times(t.suite)
         time = -t.result.elapsed if t.isFailure() else t.result.elapsed
         # The "path" here is only used as a key into a dictionary. It is never
         # used as an actual path to a filesystem API, therefore we use '/' as
         # the canonical separator so that Unix and Windows machines can share
         # timing data.