gimple-fold: Fix the recent ifcombine optimization for _BitInt [PR118023]

The BIT_FIELD_REF verifier has:
          if (INTEGRAL_TYPE_P (TREE_TYPE (op))
              && !type_has_mode_precision_p (TREE_TYPE (op)))
            {
              error ("%qs of non-mode-precision operand", code_name);
              return true;
            }
check among other things, so one can't extract something out of say
_BitInt(63) or _BitInt(4096).
The new ifcombine optimization happily creates such BIT_FIELD_REFs
and ICEs during their verification.

The following patch fixes that by rejecting those in decode_field_reference.

2024-12-14  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/118023
	* gimple-fold.cc (decode_field_reference): Return NULL_TREE if
	inner has non-type_has_mode_precision_p integral type.

	* gcc.dg/bitint-119.c: New test.