[MachineBasicBlock] Add a non-assert live-in accessor for debug mode.
[MachineBasicBlock] Add a non-assert live-in accessor for debug mode.

With r291169, it is now not possible to access the live-in information
when the liveness is not properly tracked. Although this is want we want
in general, for debugging purpose we may want to still be able to
traverse this information even if it may not be accurate.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@291317 91177308-0d34-0410-b5e6-96231b3b80d8

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