tree-ssa: Fix ICE in build_vector_type [PR93780]

The following testcase ICEs, because execute_update_addresses_taken attempts
to create a VECTOR_TYPE with non-power of 2 number of elts.
Fixed by guarding it with the corresponding predicate.

2020-02-18  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/93780
	* tree-ssa.c (non_rewritable_lvalue_p): Check valid_vector_subparts_p
	before calling build_vector_type.
	(execute_update_addresses_taken): Likewise.

	* gcc.dg/pr93780.c: New test.