[globalisel] Separate the SelectionDAG importer from the emitter. NFC
Summary:
In the near future the rules will be sorted between these two steps to
ensure that more important rules are not prevented by less important ones.
Reviewers: t.p.northover, ab, rovka, qcolombet, aditya_nandakumar
Reviewed By: ab
Subscribers: dberris, kristof.beyls, llvm-commits
Differential Revision: https://reviews.llvm.org/D29709
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@295661
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