Fix markup in gcc.dg/vect/vect-cond-reduc-3.c

Fix markup in gcc.dg/vect/vect-cond-reduc-3.c

gcc.dg/vect/vect-cond-reduc-3.c had been failing on
arm-linux-gnueabihf since the test was added, because the test needs
support for VEC_COND_EXPR <float cmp float, int, int> whereas the target
only supports VEC_COND_EXPRs in which all modes are the same.  (I have
a fix for that, but it's not really stage 3 material.)

2019-11-22  Richard Sandiford  <richard.sandiford@arm.com>

gcc/testsuite/
	* gcc.dg/vect/vect-cond-reduc-3.c: Require vect_cond_mixed
	rather than vect_condition.

From-SVN: r278612