onWrite now sets the dependency to make sure the order of setting
dependency and adding memory barrier is correct.  onReadByBuffer is
added to handle buffer-to-buffer dependency and barrier setting
correctly without causing a graph loop.  onExternalWrite is added so
that BufferVk doesn't have to track write access flags.

Additionally, setting write dependencies now include both read and write
flags.  This is in preparation for SSBO support where the buffer can be
used to read data in addition to write.

Bug: angleproject:3561
Change-Id: I2028186ea14459cd159cf79f6d640df54538fc62
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1687119Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>