Check for a supported comparison when using EXTRACT_LAST_REDUCTION

The EXTRACT_LAST_REDUCTION handling needs to generate a separate
comparison instruction that feeds the vector mask argument of the
IFN_EXTRACT_LAST call.  We weren't checking whether that comparison
was supported, leading to an ICE on the testcase.

2019-12-29  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-stmts.c (vectorizable_condition): For extract-last
	reductions, check that the target supports the required comparison
	operation.

gcc/testsuite/
	* gcc.dg/vect/vect-cond-12.c: New test.

From-SVN: r279752