Follow up to 260439, Speculative fix to clang builders

It looks like clang has a couple of test cases which caught the fact LVI was not slightly more precise after 260439.  When looking at the failures, it struck me as wasteful to be querying nullness of a constant via LVI, so instead of tweaking the clang tests, let's just stop querying constants from this source.

llvm-svn: 260451