Add missing VECTOR_MODE_P checks (PR 92595)

Add missing VECTOR_MODE_P checks (PR 92595)

This patch fixes some cases in which we weren't checking whether we had
a vector mode before calling related_vector_mode or before making vector
optab queries.

2019-11-21  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR tree-optimization/92595
	* tree-vect-stmts.c (get_group_load_store_type): Add a VECTOR_MODE_P
	check.
	(vectorizable_store, vectorizable_load): Likewise.

gcc/testsuite/
	PR tree-optimization/92595
	* g++.dg/vect/pr92595.cc: New test.

From-SVN: r278590