[LoopUnrollRuntime] Use SCEV exit count for calculating trip count. NFCI
[LoopUnrollRuntime] Use SCEV exit count for calculating trip count. NFCI

Instead of getBackEdgeTakenCount, use getExitCount on the latch exiting block
(which is proven to be the only exiting block in the loop to be unrolled).

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@306410 91177308-0d34-0410-b5e6-96231b3b80d8

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