GlobalISel: Define GINodeEquiv for undef
GlobalISel: Define GINodeEquiv for undef

AMDGPU uses this for undef vector elements in some patterns which will
be enabled in a future patch.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@370918 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm