Implementation for packHalf2x16, unpackHalf2x16 intrinsic functions.

Change-Id: I55212f8bc2ecd30e0108858d74117c3cf60733ed
Reviewed-on: https://swiftshader-review.googlesource.com/5056Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>