lower-bitint: Fix handle_cast ICE [PR113102]

lower-bitint: Fix handle_cast ICE [PR113102]

My recent change to use m_data[save_data_cnt] instead of
m_data[save_data_cnt + 1] when inside of a loop (m_bb is non-NULL)
broke the following testcase.  When we create a PHI node on the loop
using prepare_data_in_out, both m_data[save_data_cnt{, + 1}] are
computed and the fix was right, but there are also cases when we in
a loop (m_bb non-NULL) emit a nested cast with too few limbs and
then just use constant indexes for all accesses - in that case
only m_data[save_data_cnt + 1] is initialized and m_data[save_data_cnt]
is NULL.  In those cases, we want to use the former.

2023-12-22  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/113102
	* gimple-lower-bitint.cc (bitint_large_huge::handle_cast): Only
	use m_data[save_data_cnt] if it is non-NULL.

	* gcc.dg/bitint-58.c: New test.