Add missing target check for fully-masked fold-left reductions

The fold-left reduction code has a (rarely-used) fallback that handles
cases in which the loop is fully-masked and the target has no native
support for the reduction.  The fallback includea a VEC_COND_EXPR
between the reduction vector and a safe value, so we should check
whether that VEC_COND_EXPR is supported.

2019-12-27  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-loop.c (vectorizable_reduction): Check whether the
	target supports the required VEC_COND_EXPR operation before
	allowing the fallback handling of masked fold-left reductions.

gcc/testsuite/
	* gcc.target/aarch64/sve/mixed_size_10.c: New test.

From-SVN: r279742