r300: merge simple movs with constant swizzles together

r300: merge simple movs with constant swizzles together

This pass will merge instructions like these

MOV output[0].x, temp[5].x___;
MOV output[0].yzw, none._001;

into

MOV output[0].xyzw, temp[5].x001;

It is currently very careful with control flow and dependency
tracking, so there is still room for improvements.

Shader-db stats with RV530:
total instructions in shared programs: 132486 -> 132256 (-0.17%)
instructions in affected programs: 6186 -> 5956 (-3.72%)
helped: 65
HURT: 0
total temps in shared programs: 18035 -> 18014 (-0.12%)
temps in affected programs: 295 -> 274 (-7.12%)
helped: 22
HURT: 1

Signed-off-by: Pavel Ondračka <pavel.ondracka@gmail.com>
Reviewed-by: Filip Gawin <filip@gawin.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16657>