GVN-hoist: use DFS numbers instead of walking the instruction stream
GVN-hoist: use DFS numbers instead of walking the instruction stream

The patch replaces a function that walks the IR with a call to firstInBB() that
uses the DFS numbering.  NFC.

Differential Revision: https://reviews.llvm.org/D22809

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@276840 91177308-0d34-0410-b5e6-96231b3b80d8

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