[LVI] Check for @llvm.assume dominating the edge branch

When LazyValueInfo uses @llvm.assume intrinsics to provide edge-value
constraints, we should check for intrinsics that dominate the edge's branch,
not just any potential context instructions. An assumption that dominates the
edge's branch represents a truth on that edge. This is specifically useful, for
example, if multiple predecessors assume a pointer to be nonnull, allowing us
to simplify a later null comparison.

The test case, and an initial patch, were provided by Philip Reames. Thanks!
Fix dotest.py test runner exit code to return non-zero on failure/error.

This addresses this bug:
http://www.llvm.org/bugs/show_bug.cgi?id=21267

Which has been broken since svn r215256 on Aug 8 2014.

DO NOT REVERT THIS COMMIT EVEN IF IT CREATES TEST FAILURES.

The test failures are a result of accumulation of hidden failures
that were masked by the bug this change fixes.  Most of our test
runners as part of build testing rely on dotest.py returning non-zero to indicate
some kind of errant test run.  Thus, we have been flying blind
since Aug 8 2014.