RegisterCoalescer: Need to check DstReg+SrcReg for missing undef flags
RegisterCoalescer: Need to check DstReg+SrcReg for missing undef flags

copy coalescing with enabled subregister liveness can reveal undef uses,
previously this was only checked for the SrcReg in updateRegDefsUses()
but we need to check DstReg as well.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@262767 91177308-0d34-0410-b5e6-96231b3b80d8

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