[LoopUnroll] Don't clear out the AssumptionCache on each loop
[LoopUnroll] Don't clear out the AssumptionCache on each loop

Clearing out the AssumptionCache can cause us to rescan the entire
function for assumes.  If there are many loops, then we are scanning
over the entire function many times.

Instead of clearing out the AssumptionCache, register all cloned
assumes.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@278854 91177308-0d34-0410-b5e6-96231b3b80d8

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