aco: Don't generate an if when the first part of a merged HS or GS is empty.

aco: Don't generate an if when the first part of a merged HS or GS is empty.

In some cases (eg. in a few tessellation CTS tests) the VS part of
a merged HS is completely empty. Let's not generate a divergent if
in these cases. (LLVM also doesn't do it.)

No pipeline DB changes, only affects the CTS.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3964>