Minor refactor of arithmetic codegen.
Minor refactor of arithmetic codegen.

Merged down to a single case per build_* variant.

Former-commit-id: d8c99b49bf6b5243669593d7440fa0bb0678341c
Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide