nvptx: Fix ICE in nvptx_vector_alignment on gcc.dg/attr-vector_size.c

nvptx: Fix ICE in nvptx_vector_alignment on gcc.dg/attr-vector_size.c

This patch addresses the ICE in gcc.dg/attr-vector_size.c during
make -k check on nvptx-none.  The actual ICE looks like:

testsuite/gcc.dg/attr-vector_size.c:29:1: internal compiler error: \
  in tree_to_shwi, at tree.c:7321
0xf53bf2 tree_to_shwi(tree_node const*)
../../gcc/gcc/tree.c:7321
0xff1969 nvptx_vector_alignment
../../gcc/gcc/config/nvptx/nvptx.c:5105^M

The problem is that the caller has ensured that TYPE_SIZE(type) is
representable as an unsigned HOST_WIDE_INT, but nvptx_vector_alignment is
accessing it as a signed HOST_WIDE_INT which overflows in pathological
conditions.  Amongst those pathological conditions is that a TYPE_SIZE of
zero can sometimes reach this function, prior to an error being emitted.
Making sure the result is not less than the mode's alignment and not greater
than BIGGEST_ALIGNMENT fixes the ICEs, and generates the expected
compile-time error messages.

Tested on --target=nvptx-none, with a "make" and "make check" which results
in four fewer unexpected failures and three more expected passes.

2020-07-03  Roger Sayle  <roger@nextmovesoftware.com>
	    Tom de Vries  <tdevries@suse.de>

gcc/ChangeLog:

	PR target/90932
	* config/nvptx/nvptx.c (nvptx_vector_alignment): Use tree_to_uhwi
	to access TYPE_SIZE (type).  Return at least the mode's alignment.