Fix bugs in CodeGen_GPU_Host.cpp (not always passing user context)
Fix bugs in CodeGen_GPU_Host.cpp (not always passing user context)


Former-commit-id: 43ffef71bad5e49dfced2592a12d08346e61fd22
Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide