Don't convert size-1 GPU for loops to if statements.
Don't convert size-1 GPU for loops to if statements.

E.g. the loop over thread indices


Former-commit-id: d4a19db9296dc2d3709d246c3a90e77e303ab615
darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide