[globalisel] Separate the SelectionDAG importer from the emitter. NFC
[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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@295661 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm