- Support vectorized casts

- It is incorrect to extract elements from int8x4 with

   0x000000ff & (x >> i * 8)

  as this value is of type int in C/C++. If this expression
  is used for sign extensions, the sign bit will be wrong.
  Simply use C style casts instead and sign bits will just work.

Signed-off-by: Wei Pan <weip@nvidia.com>