Vendor in the ir module piecemeal

Becuase the IR module is interdependent with so many other modules,
it will be vendored in without being fully adopted inside numba-cuda.
Modules inside numba-cuda will not use numba.cuda.core.ir yet.
In a subsequent PR when other components are vendored in, we may
update these imports. For now, its only use is in test_ir.py.

Author: ashermancinelli