lower-bitint: Fix lowering of non-_BitInt to _BitInt cast merged with some wider...

lower-bitint: Fix lowering of non-_BitInt to _BitInt cast merged with some wider cast [PR112940]

The following testcase ICEs, because a PHI argument from latch edge
uses a SSA_NAME set only in a conditionally executed block inside of the
loop.
This happens when we have some outer cast which lowers its operand several
times, under some condition with variable index, under different condition
with some constant index, otherwise something else, and then there is
an inner cast from non-_BitInt integer (or small/middle one).  Such cast
in certain conditions is emitted by initializing some SSA_NAMEs in the
initialization statements before loops (say for casts from <= limb size
precision by computing a SSA_NAME for the first limb and then extension
of it for the later limbs) and uses the prepare_data_in_out function
to create a PHI node.  Such function is passed the value (constant or
SSA_NAME) to use in the PHI argument from the pre-header edge, but for
the latch edge it always created a new SSA_NAME and then caller emitted
in the following 3 spots an extra assignment to set that SSA_NAME to
whatever value we want from the latch edge.  In all these 3 cases
the argument from the latch edge is known already before the loop though,
either constant or SSA_NAME computed in pre-header as well.
But the need to emit an assignment combined with the handle_operand done
in a conditional basic block results in the SSA verification failure.

The following patch fixes it by extending the prpare_data_in_out method,
so that when the latch edge argument is known before (constant or computed
in pre-header), we can just use it directly and avoid the extra assignment
that would normally be hopefully optimized away later to what we now emit
directly.

2023-12-13  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/112940
	* gimple-lower-bitint.cc (struct bitint_large_huge): Add another
	argument to prepare_data_in_out method defaulted to NULL_TREE.
	(bitint_large_huge::handle_operand): Pass another argument to
	prepare_data_in_out instead of emitting an assignment to set it.
	(bitint_large_huge::prepare_data_in_out): Add VAL_OUT argument.
	If non-NULL, use it as PHI argument instead of creating a new
	SSA_NAME.
	(bitint_large_huge::handle_cast): Pass rext as another argument
	to 2 prepare_data_in_out calls instead of emitting assignments
	to set them.

	* gcc.dg/bitint-53.c: New test.