Don't try to vectorise COND_EXPR reduction chains (PR 84913)

Don't try to vectorise COND_EXPR reduction chains (PR 84913)

The testcase ICEd for both SVE and AVX512 because we were trying
to vectorise a chain of COND_EXPRs as a reduction and getting
confused by reduc_index == -1.

2018-03-18  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR tree-optimization/84913
	* tree-vect-loop.c (vectorizable_reduction): Don't try to
	vectorize chains of COND_EXPRs.

gcc/testsuite/
	PR tree-optimization/84913
	* gfortran.dg/vect/pr84913.f90: New test.

From-SVN: r258631