[TargetLowering] SimplifyDemandedBits - rename demanded mask args. NFCI.
[TargetLowering] SimplifyDemandedBits - rename demanded mask args. NFCI.

Help stop bugs like rL343935 by making the 'original' DemandedBits arg more obviously not the mask that is actually used.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@344138 91177308-0d34-0410-b5e6-96231b3b80d8

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