[ValueTracking] Teach GetUnderlyingObject to stop when it reachs an alloca instruction.
[ValueTracking] Teach GetUnderlyingObject to stop when it reachs an alloca instruction.

Previously it tried to call SimplifyInstruction which doesn't know anything about alloca so defers to constant folding which also doesn't do anything with alloca. This results in wasted cycles making calls that won't do anything. Given the frequency with which this function is called this time adds up.



git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@300118 91177308-0d34-0410-b5e6-96231b3b80d8

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