Move all allocas to the top of the function
Move all allocas to the top of the function

This means we don't need to do explicit stacksave/stackrestore. This makes
foo.compute_at(bar, x) faster, because it includes less stack-related junk.


Former-commit-id: 0a2790ff39ddd21406842d398a0ef13939717af1
darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide