Hexagon (target/hexagon) assignment to c4 should wait until packet commit

On Hexagon, c4 is an alias for predicate registers P3:0.  If we assign to
c4 inside a packet with reads from predicate registers, the predicate
reads should get the old values.

Test case added to tests/tcg/hexagon/preg_alias.c

Co-authored-by: Michael Lambert <mlambert@cuicinc.com>
Signed-off-by: Taylor Simpson <tsimpson@quicinc.com>
Message-Id: <20220210021556.9217-13-tsimpson@quicinc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>