lower-bitint: Fix up handling of nested casts in mergeable stmt handling [PR112941]

lower-bitint: Fix up handling of nested casts in mergeable stmt handling [PR112941]

The following patch fixes 2 issues in handling of casts for mergeable
stmts.
The first hunk fixes the case when we have two nested casts (typically
after optimization that is zero-extension of a sign-extension because
everything else should have been folded into a single cast).  If
the lowering of the outer cast needs to make the code conditional
(e.g.
  for (...)
    {
      if (idx <= 32)
        {
          if (idx < 32)
            { ... handle_operand (idx); ... }
          else
            { ... handle_operand (32); ... }
        }
      ...
    }
) and the lowering of the inner one as well, right now it creates invalid
SSA form, because even for the inner cast we need a PHI on the loop
and the PHI argument from the latch edge is a SSA_NAME initialized in
the conditionally executed bb.  The hunk fixes that by detecting such
a case and adding further PHI nodes at the end of the ifs such that
the right value propagates to the next loop iteration.  We can use
0 arguments for the other edges because the inner operand handling
is only done for the first set of iterations and then the other ifs take
over.

The rest fixes a case of again invalid SSA form, when for a sign extension
we need to use the 0 or -1 value initialized by earlier iteration in
a constant idx case, the code was using the value of the loop PHI argument
from latch edge rather than result; that is correct for cases expanded
in straight line code after the loop, but not inside of the loop for the
cases of handle_cast conditionals, there we should use PHI result.  This
is done in the second hunk and supported by the remaining hunks, where
it clears m_bb to tell the code we aren't in the loop anymore.

Note, this patch doesn't deal with similar problems during multiplication,
division, floating casts etc. where we just emit a library call.  I'll
need to make sure in that case we don't merge more than one cast per
operand.

2023-12-20  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/112941
	* gimple-lower-bitint.cc (bitint_large_huge::handle_cast): If
	save_cast_conditional, instead of adding assignment of t4 to
	m_data[save_data_cnt + 1] before m_gsi, add phi nodes such that
	t4 propagates to m_bb loop.  For constant idx, use
	m_data[save_data_cnt] rather than m_data[save_data_cnt + 1] if inside
	of the m_bb loop.
	(bitint_large_huge::lower_mergeable_stmt): Clear m_bb when no longer
	expanding inside of that loop.
	(bitint_large_huge::lower_comparison_stmt): Likewise.
	(bitint_large_huge::lower_addsub_overflow): Likewise.
	(bitint_large_huge::lower_mul_overflow): Likewise.
	(bitint_large_huge::lower_bit_query): Likewise.

	* gcc.dg/bitint-55.c: New test.