r300: merge MOVs into ADD using the 0 swizzle

r300: merge MOVs into ADD using the 0 swizzle

Shader-db stats with RV530:
total instructions in shared programs: 169509 -> 166013 (-2.06%)
instructions in affected programs: 99126 -> 95630 (-3.53%)
total presub in shared programs: 10975 -> 10758 (-1.98%)
presub in affected programs: 744 -> 527 (-29.17%)
total temps in shared programs: 21722 -> 21649 (-0.34%)
temps in affected programs: 1350 -> 1277 (-5.41%)

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/17560>