PR middle-end/90597: gcc_assert ICE in layout_type

PR middle-end/90597: gcc_assert ICE in layout_type

This patch fixes the default implementation of TARGET_VECTOR_ALIGNMENT,
known as default_vector_alignment, using the same logic as my earlier
nvptx patch, as the ICE caused by TYPE_SIZE(type) being zero during
error handling in gcc.dg/attr-vector_size.c is common among backends,
and is known in bugzilla as PR middle-end/90597, apparently a recent
regression.

2020-09-01  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog:
	PR middle-end/90597
	* targhooks.c (default_vector_alignment): Return at least the
	GET_MODE_ALIGNMENT for the type's mode.