[tablegen][globalisel] Convert the SelectionDAG importer to a tree walking approach. NFC

Summary:
But don't actually inspect the tree any deeper than we already do. This
change is NFC but the next one will enable full traversal of the
source/destination patterns.

Depends on D30535

Reviewers: t.p.northover, qcolombet, aditya_nandakumar, rovka, ab

Subscribers: igorb, dberris, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D30536

llvm-svn: 299001