[Dominators] Clean up typedefs in GenericDomTreeConstruction. NFC.
[Dominators] Clean up typedefs in GenericDomTreeConstruction. NFC.

Summary: This patch cleans up GenericDomTreeConstruction by replacing typedefs with usings and replaces `typename GraphT::NodeRef` with `NodePtr` to make the file more readable.

Reviewers: sanjoy, dberlin, chandlerc

Reviewed By: dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34254

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@305715 91177308-0d34-0410-b5e6-96231b3b80d8

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