[BDCE] Add tests for PR39771; NFC
[BDCE] Add tests for PR39771; NFC

These involve cases where certain uses are dead by means of having
no demanded bits, even though the used instruction still has demanded
bits when other uses are taken into account. BDCE currently does not
simplify such cases.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@348861 91177308-0d34-0410-b5e6-96231b3b80d8

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