nir/nir_lower_wrmasks: Use the nir_lower_instructions_pass() helper.

This fixes the invalidation of metadata when we didn't modify the shader
and unindents a bunch of code.

Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6412>
1 file changed