Fixed constant folding through let statements to not remove them.
Fixed constant folding through let statements to
not remove them.

Also fixed some unused folding rules.

Former-commit-id: a340962132d9190e7a3a34e0ac676db1359247b3
darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide