Fix invalid assumption in vect_transform_stmt (PR 86871)

Fix invalid assumption in vect_transform_stmt (PR 86871)

The handling of outer-loop uses of inner-loop definitions assumed
that anything that wasn't a PHI would be a gassign.  It's also
possible for it to be a gcall.

2018-08-08  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR tree-optimization/86871
	* tree-vect-stmts.c (vect_transform_stmt): Use gimple_get_lhs
	instead of gimple_assign_lhs.

gcc/testsuite/
	PR tree-optimization/86871
	* gcc.dg/vect/pr86871.c: New test.

From-SVN: r263447