Fix exponential behavior in bounds inference
Fix exponential behavior in bounds inference

Deeply nested let expressions were causing problems in the same way let
statements used to because we were effectively substituting them in. The
fix was similar.


Former-commit-id: 871c7321b7fceff2c9de2b86173f7a7a4ee2261d
Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide