This change includes preparatory work for textureGatherOffsets as well,
though that requires special handling which is addressed in a follow-up
change.  This special handling is due to the fact that this family of
functions is the only built-in with an array parameter, and our built-in
table generation doesn't handle it correctly.

Bug: angleproject:3569
Change-Id: Ic1c8dd03878a184be00f578ab9fc42af8f6e47ea
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1965435Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jiajia Qin <jiajia.qin@intel.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>