ScheduleDAGInstrs: Do not try to toggle kill flags on debug uses
ScheduleDAGInstrs: Do not try to toggle kill flags on debug uses

Preparation for upcoming changes. No testcase as none of the public
targets bundles early enough and has a post machine scheduler enabled at
the same time. The error is also easily catched by asserts.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@293324 91177308-0d34-0410-b5e6-96231b3b80d8

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