Implementation for packSnorm2x16, unpackSnorm2x16, packUnorm2x16 and
unpackUnorm2x16 intrinsic functions.

Change-Id: I6b9e2584c1aaad8011f026c217d8ad3f72e9ba45
Reviewed-on: https://swiftshader-review.googlesource.com/5053Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>