brw: Add explicit way to get an empty brw_builder

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>

src/intel/compiler/test_opt_combine_constants.cpp

@@ -68,7 +68,7 @@ struct FSCombineConstantsTest : public ::testing::Test {
 static brw_builder
 make_builder(brw_shader *s)
 {
-   return brw_builder(s, s->dispatch_width).at_end();
+   return brw_builder(s).at_end();
 }
 
 TEST_F(FSCombineConstantsTest, Simple)