Even though it is not common usage case that the same texture is used as
attachment and texture, but aztec ruins is using it. And earlier version
of gfxbench's T-Rex also running into this. So performance is still
important. This CL splits the texture from fragment shader usage case
out so we can have lighter barrier.

Bug: angleproject:5780
Change-Id: Ifdcbfe24488fdac62826c5af6ecfbb05f87c0499
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2795269
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>