aco: Use more optimal sequence at the beginning of merged shaders.

aco: Use more optimal sequence at the beginning of merged shaders.

It can be further optimized in the future, but
the new sequence already has a few advantages:

* Uses fewer instructions
* Uses even fewer instructions in wave32 mode
* Doesn't use the VALU at all

Totals from affected shaders (GFX10):
VGPRS: 43504 -> 43496 (-0.02 %)
Code Size: 2436000 -> 2423688 (-0.51 %) bytes
Max Waves: 8704 -> 8705 (0.01 %)

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4165>