Remove a magic number
Remove a magic number

Another source of guilt... I was using a magic -1 to represent let
stmts. This PR added a -2. Time for an enum.


Former-commit-id: 49ebefab0c3fca258c2ebbb48623271e1a2f2f25
Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide