[Hexagon] Custom lower VECTOR_SHUFFLE and EXTRACT_SUBVECTOR for HVX

If the mask of a vector shuffle has alternating odd or even numbers
starting with 1 or 0 respectively up to the largest possible index
for the given type in the given HVX mode (single of double) we can
generate vpacko or vpacke instruction respectively.

E.g.
  %42 = shufflevector <32 x i16> %37, <32 x i16> %41,
                      <32 x i32> <i32 1, i32 3, ..., i32 63>
  is %42.h = vpacko(%41.w, %37.w)

Patch by Pranav Bhandarkar.

llvm-svn: 277168