lower-bitint: Fix up -fnon-call-exceptions bit-field load lowering [PR112668]

lower-bitint: Fix up -fnon-call-exceptions bit-field load lowering [PR112668]

As the following testcase shows, there are some bugs in the
-fnon-call-exceptions bit-field load lowering.  In particular, there
is a case where we want to emit a load early in the initialization
(before m_init_gsi) and because that load might throw exception, need
to split block after the load so that it has an EH edge.
Now, across this splitting, we have m_init_gsi, save_gsi (something
we put back into m_gsi afterwards) statement iterators and m_preheader_bb
which is used to determine the pre-header edge of a loop (if any).
As the testcase shows, both of these statement iterators and m_preheader_bb
as well need adjustments if the block was split.  If the stmt iterators
refer to a statement, they need to be updated so that if the statement is
in the bb after the split gsi_bb and gsi_seq is updated, otherwise they
ought to be the start of the new (second) bb.
Similarly, m_preheader_bb should be updated to the second bb if it was
the first before.  Other spots where we insert something before m_init_gsi
don't split blocks in there and are fine.

The m_gsi iterator is normal iterator to insert statements before it,
so gsi_end_p means insert statements at the end of basic block.
m_init_gsi is on the other side an iterator after which statements should be
inserted (so gsi_end_p means insert statements at the start of basic block
after labels), but the whole pass is written for insertion of statements before
iterators, so when in 3 spots it wants to insert something after m_init_gsi,
it saves current iterator to save_gsi and sets m_gsi to gsi_after_labels
if m_init_gsi was gsi_end_p, or to the next statement.  But it actually wasn't
updating m_init_gsi back when switching to normal iterator, this patch changes
that such that further statements after m_init_gsi will appear after the
set of statements inserted before m_init_gsi.

Finally, the pass had a couple of places where it wanted to create a gsi_end_p
iterator for a particular basic block, instead of doing
m_gsi = gsi_last_bb (bb); if (!gsi_end_p (m_gsi)) gsi_next (&m_gsi);
the pass now uses new m_gsi = gsi_end_bb (bb) function.

2023-11-24  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/112668
	* gimple-iterator.h (gsi_end, gsi_end_bb): New inline functions.
	* gimple-lower-bitint.cc (bitint_large_huge::handle_cast): After
	temporarily adding statements after m_init_gsi, update m_init_gsi
	such that later additions after it will be after the added statements.
	(bitint_large_huge::handle_load): Likewise.  When splitting
	gsi_bb (m_init_gsi) basic block, update m_preheader_bb if needed
	and update saved m_gsi as well if needed.
	(bitint_large_huge::lower_mergeable_stmt,
	bitint_large_huge::lower_comparison_stmt,
	bitint_large_huge::lower_mul_overflow,
	bitint_large_huge::lower_bit_query): Use gsi_end_bb.

	* gcc.dg/bitint-40.c: New test.