And use brw_builder(brw_shader *) and brw_builder() constructors
where possible.
The way tests are written, it is necessary to initialize an "empty"
builder -- which is later replaced by a proper one. Default parameter
NULL make that initialization implicit.
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33815>