Module Ast.Narrow2

Witness types for narrowing to upper half (XTN2, etc). 16B->8H, 8H->4S, 4S->2D

type ('src_arr, 'src_w, 'dst_arr, 'dst_w) t =
  1. | V16B_V8H : ([ `V8H ], [ `H ], [ `V16B ], [ `B ]) t
  2. | V8H_V4S : ([ `V4S ], [ `S ], [ `V8H ], [ `H ]) t
  3. | V4S_V2D : ([ `V2D ], [ `D ], [ `V4S ], [ `S ]) t