GlobalISel: Move finalizeLowering call later

This matches the DAG behavior where this is called after the loop
checking for calls. The AMDGPU implementation depends on knowing if
there are calls in the function or not, so move this later.

Another problem is finalizeLowering is actually called twice; I was
seeing weird inconsistencies since the first call would produce
unexpected results and the second run would correct them in some
contexts. Since this requires disabling the verifier, and it's useful
to serialize the MIR immediately after selection, FinalizeISel should
probably not be a real pass.