expr: Move reduce_bit_field target mode check [PR96151]

expr: Move reduce_bit_field target mode check [PR96151]

In some cases, expand_expr_real_2 prefers to use the mode of the
caller-suggested target instead of the mode of the expression when
passing values to reduce_to_bit_field_precision.  E.g.:

      else if (target == 0)
        op0 = convert_to_mode (mode, op0,
                               TYPE_UNSIGNED (TREE_TYPE
                                              (treeop0)));
      else
        {
          convert_move (target, op0,
                        TYPE_UNSIGNED (TREE_TYPE (treeop0)));
          op0 = target;
        }

where “op0” might not have “mode” for the “else” branch,
but does for all the others.

reduce_to_bit_field_precision discards the suggested target if it
has the wrong mode.  This patch moves that to expand_expr_real_2
instead (conditional on reduce_bit_field).

gcc/
	PR middle-end/96151
	* expr.c (expand_expr_real_2): When reducing bit fields,
	clear the target if it has a different mode from the expression.
	(reduce_to_bit_field_precision): Don't do that here.  Instead
	assert that the target already has the correct mode.