[Hexagon] Introduce noop intrinsic to cast between vector predicate types

The (overloaded) intrinsic is llvm.hexagon.V6.pred.typecast[.128B]. The
types of the operand and the return value are HVX boolean vector types.
For each cast, there needs to be a corresponding intrinsic declared,
with different suffixes appended to the name, e.g.
  ; cast <128 x i1> to <32 x i1>
  declare <32 x i1> @llvm.hexagon.V6.pred.typecast.128B.s1(<128 x i1>)
  ; cast <32 x i1> to <64 x i1>
  declare <64 x i1> @llvm.hexagon.V6.pred.typecast.128B.s2(<32 x i1>)
etc.