lower-bitint: Fix up handling of INTEGER_CSTs in handle_operand in right shifts or...

lower-bitint: Fix up handling of INTEGER_CSTs in handle_operand in right shifts or comparisons [PR113370]

The INTEGER_CST code uses the remainder bits in computations whether to use
whole constant or just part of it and extend it at runtime, and furthermore
uses it to avoid using all bits even when using the (almost) whole constant.
The problem is that the prec % (2 * limb_prec) computation it uses is
appropriate only for the normal lowering of mergeable operations (where
we process 2 limbs at a time in a loop starting with least significant
limbs and process the remaining 0-2 limbs after the loop (there with
constant indexes).  For that case it is ok not to emit the upper
prec % (2 * limb_prec) bits into the constant, because those bits will be
extracted using INTEGER_CST idx and so will be used directly in the
statements as INTEGER_CSTs.
For other cases, where we either process just a single limb in a loop,
process it downwards (e.g. non-equality comparisons) or with some runtime
addends (some shifts), there is either just at most one limb lowered with
INTEGER_CST idx after the loop (e.g. for right shift) or before the loop
(e.g. non-equality comparisons), or all limbs are processed with
non-INTEGER_CST indexes (e.g. for left shift, when m_var_msb is set).
Now, the m_var_msb case is already handled through
              if (m_var_msb)
                type = TREE_TYPE (op);
              else
                /* If we have a guarantee the most significant partial limb
                   (if any) will be only accessed through handle_operand
                   with INTEGER_CST idx, we don't need to include the partial
                   limb in .rodata.  */
                type = build_bitint_type (prec - rem, 1);
but for the right shifts or comparisons the prec - rem when rem was
prec % (2 * limb_prec) was incorrect, so the following patch fixes it
to use remainder for 2 limbs only if m_upwards_2limb and remainder for
1 limb otherwise.

2024-01-15  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/113370
	* gimple-lower-bitint.cc (bitint_large_huge::handle_operand): Only
	set rem to prec % (2 * limb_prec) if m_upwards_2limb, otherwise
	set it to just prec % limb_prec.

	* gcc.dg/torture/bitint-48.c: New test.