[LoopVectorize] Add overflow checks when tail-folding with scalable vectors

In InnerLoopVectorizer::getOrCreateVectorTripCount there is an
assert that the known minimum value for the VF is a power of 2
when tail-folding is enabled. However, for scalable vectors the
value of vscale may not be a power of 2, which means we have
to worry about the possibility of overflow. I have solved this
problem by adding preheader checks that prevent us from entering
the vector body if the canonical IV would overflow, i.e.

  if ((IntMax - TripCount) < (VF * UF)) ... skip vector loop ...

Differential Revision: https://reviews.llvm.org/D125235