[Loop Vectorizer] Abandon vectorization when no integer IV found

Support for vectorizing loops with secondary floating-point induction
variables was added in r276554.  A primary integer IV is still required
for vectorization to be done.  If an FP IV was found, but no integer IV
was found at all (primary or secondary), the attempt to vectorize still
went forward, causing a compiler-crash.  This change abandons that
attempt when no integer IV is found.  (Vectorizing FP-only cases like
this, rather than bailing out, is discussed as possible future work
in D52327.)

See PR38800 for more information.

Differential Revision: https://reviews.llvm.org/D52327

llvm-svn: 342786