Remove some code in prefetch that looked unnecessary
Remove some code in prefetch that looked unnecessary

Massive speedup in lowering for all pipelines (3x for local laplacian).

I don't see why we need to expand prefetch bounds based on *outer* lets
and loops -- we're not lifting code outside of them. box_touched at the
right scope should do the right thing for us.


Former-commit-id: 2464116059ed3dfc35a71be8ba0a874edec16c16 [formerly 8d71c53cded9c7c45c9ea1eb6851176ac45b34ce]
Former-commit-id: d8fa9f70e0b4b786283b69252bb8c01ab924c9e8
darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide