Added tests for (and fixed) let simplification rules
Added tests for (and fixed) let simplification rules


Former-commit-id: 08cafa96919c1112a7d3c71553d309491b351b2f
Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide