ContextVk::onBufferRead can check for "usesBufferForWrite".

Bug: angleproject:4911
Change-Id: Id295c068fd9a4027c3127830c682d62d614718dc
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2355102Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Tim Van Patten <timvp@google.com>