If an address expression is given to `to_amode` that is completely
constant (no registers at all), then it will produce an `Amode` that has
the resulting constant as an offset, and `(invalid_reg)` as the base.
This is a side-effect of the way we build up the amode step-by-step --
we're waiting to see a register and plug it into the base field. If we
never get a reg though, we need to generate a constant zero into a
register and use that as the base. This PR adds a `finalize_amode`
helper to do just that.
Fixes#4234.
54acd8b3e2