Minor fix to ARM codegen. Missed an error in the last commit.
Created an interleave_vectors method in the codegen class that implements the interleave_vectors intrinsic call in halide. This was necessary so that I could use a recursive approach to define interleave_vectors for any number of arguments. For calls to interleave_vectors with 1 to 4 args specialized llvm code is generated. When more args are present we recursively interleave the even args as well as the odd args, and then interleave the results of these.