intel/compiler: Move details of NIR conversion out of fs_visitor
Make most of the functions used in the NIR conversion just statics as part of brw_fs_nir.cpp. The data used by NIR conversion is also localized and is available just during that operation.
And at the end of the series, we also get rid of fs_visitor::bld field, making the builder depend on the shader and not the other way around.
On top of !26301 (merged) and !26302 (merged).