Hexagon (target/hexagon) Short-circuit packet HVX writes

Hexagon (target/hexagon) Short-circuit packet HVX writes

In certain cases, we can avoid the overhead of writing to future_VRegs
and write directly to VRegs.  We consider HVX reads/writes when computing
ctx->need_commit.  Then, we can early-exit from gen_commit_hvx.

Signed-off-by: Taylor Simpson <tsimpson@quicinc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230427230012.3800327-14-tsimpson@quicinc.com>