gpu.c: extract_array_info: reuse ppcg_size_from_extent

gpu.c: extract_array_info: reuse ppcg_size_from_extent

Some minor adjustments are required to be able to reuse
ppcg_size_from_extent from extract_array_info.
In particular, extract_array_info performs an extra test
to verify that there is a bound on the extent.  Move that
test into ppcg_size_from_extent.
Also, store the result of ppcg_size_from_extent directly
into gpu_array_info as an isl_multi_pw_aff.
While it would be possible to extract the isl_pw_aff objects
from the isl_multi_pw_aff inside extract_array_info,
the isl_multi_pw_aff is more convenient to work with.

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>