nir/builder: Generate the alu helpers directly in python

nir/builder: Generate the alu helpers directly in python

There's no reason for having a macro *and* a python generator.  We can
easily just do the whole thing in python.  This has the advantage that we
are no longer definining ALU# macros which conflict with the ones in
brw_fs_builder.h.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>