Hexagon (target/hexagon) Clean up pred_written usage

Hexagon (target/hexagon) Clean up pred_written usage

Only endloop instructions will conditionally write to a predicate.
When there is an endloop instruction, we preload the values into
new_pred_value.

The only place pred_written is needed is when HEX_DEBUG is on.

We remove the last use of check_for_attrib.  However, new uses will be
introduced later in this series, so we mark it with G_GNUC_UNUSED.

Signed-off-by: Taylor Simpson <tsimpson@quicinc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230427230012.3800327-9-tsimpson@quicinc.com>