[SCEV] Add some asserts on availability of arguments of isLoopEntryGuardedByCond

The logic in howManyLessThans is fishy. It first checks invariance of
RHS, and then uses OrigRHS as argument for isLoopEntryGuardedByCond, which
is, strictly saying, a different thing. We are seeing a very rare intermittent
failure of availability checks, and it looks like this precondition is
sometimes broken. Before we can figure out what's going on, adding asserts
that all involved values that may possibly to to isLoopEntryGuardedByCond
are available at loop entry.

If either of these asserts fails (OrigRHS is the most likely suspect), it
means that the logic here is flawed.