func Set(t *types.Type, off int64, bv bitvec.BitVec)
NOTE: The bitmap for a specific type t could be cached in t after the first run and then simply copied into bv at the correct offset on future calls with the same type t.
func SetNoCheck(t *types.Type, off int64, bv bitvec.BitVec)
SetNoCheck is like Set, but do not check for alignment.