r300: don't merge w channel in fragment shaders

r300: don't merge w channel in fragment shaders

Skip the merge if one of the instructions writes just w channel
and we are compiling a fragment shader. We can pair-schedule it together
later anyway and it will also give the scheduler a bit more flexibility.

Shader-db stats with RV530:
total instructions in shared programs: 169522 -> 169509 (<.01%)
instructions in affected programs: 14170 -> 14157 (-0.09%)
total temps in shared programs: 21712 -> 21722 (0.05%)
temps in affected programs: 324 -> 334 (3.09%)

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>