Add stand-in for vectorize of extern calls
Add stand-in for vectorize of extern calls

Currently raises an assertion.

Former-commit-id: ddbf9c03671fc6ca9f73be416b5dea58d812cc68
Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide