BoundaryConditions should take args by const-ref
BoundaryConditions should take args by const-ref

In most cases, it doesn’t matter; however, Input<> and Output<> types
(deliberately) don’t have copy ctors, but are convertible to Func or
Expr. Converting usage to const-ref lets them participate without overt
casting required.


Former-commit-id: 611bc1829658a221385ba42c872a4aaadbd403dc
Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide