Thank you both for your comments.

I have added the check to ensure the index vector won't cause an
overflow. I also added tests to the testsuite in order to check that the
loop is vectorized for UINT_MAX - 1 iterations but not UINT_MAX
iterations. I was not able to write code that triggers
INTEGER_INDUC_COND_REDUCTION when using char or other smaller types
(changing the types of last, min_v and a to something else causes
COND_REDUCTION to be used instead), so these tests are only compiled and
not executed.

I also moved an instruction that generates a vector of zeroes (used for
COND_REDUCTION) in the branch of code run only for COND_REDUCTION, this
should remove the unused vector that Alan noticed.

2017-11-21  Kilian Verhetsel <kilian.verhetsel@uclouvain.be>

gcc/ChangeLog:
	PR testsuite/81179
	* tree-vect-loop.c
        (vect_create_epilog_for_reduction): Fix the returned value for
	INTEGER_INDUC_COND_REDUCTION whose last match occurred at
	index 0.
	(vectorizable_reduction): For INTEGER_INDUC_COND_REDUCTION,
	pass the PHI statement that sets the induction variable to the
	code generating the epilogue and check that no overflow will
	occur.

gcc/testsuite/ChangeLog:
	* gcc.dg/vect/vect-iv-cond-reduc-overflow-1.c: New test for
        overflows when compiling conditional reductions.
        * gcc.dg/vect/vect-iv-cond-reduc-overflow-2.c: Likewise.