This was removed with a previous commit to fix behavior of invariant
pragma overall, however, the flattening is still needed for the initial
translation step of webGL VSs.

This change restores the flattening as it previously existed, however it
is now only applied to VS outputs. FS inputs are no longer flattened as
the previous fixes preclude the need for that workaround any longer.

Bug: 980675
Change-Id: Ia3c0471e24cae2905eb0d7bde153091e4d3829b4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1687788Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Tobin Ehlis <tobine@google.com>