RegAllocFast: Do not allocate registers for undef uses
RegAllocFast: Do not allocate registers for undef uses

Do not actually allocate a register for an undef use. Previously we we
would create unnecessary reload instruction for undef uses where the
register wasn't live.

Patch by Matthias Braun

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@356501 91177308-0d34-0410-b5e6-96231b3b80d8

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