The stage mask in vkCmdPipelineBarrier is incorrectly set.

Bug: angleproject:3473
Change-Id: I4fea5994a391b0db0f81183f1c4d4ba47d387acb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1631849
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>