r300: fix generation of invalid swizzles in regalloc

r300: fix generation of invalid swizzles in regalloc

rc_adjust_channels is inteded for moving the swizzles to a new channels
when rewriting the writemask of an instruction. However for readers one
needs to keep the swizzles in the old channels but rather convert to the
new values, so use the proper helper rc_rewrite_swizzle.

With the new swizzle fixed, we should properly detect that it would be
invalid and thus we can select the proper register class to prevent the
writemask rewrite in the regalloc.

Documentation was added to rc_adjust_channels to make it more clear what
it actually does.

Fixes a bunch of dEQP tests.

Cc: mesa-stable
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/7521
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/19158>