This is in preparation of textureGatherOffsets support.

Bug: angleproject:3569
Change-Id: I4ce5a1e102790181c8f843735961b9c249a69221
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1965720Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>