Provide both a performance warning and a debug-util label (for AGI)
when loading a vertex attribute requires a format conversion.

Bug: b/184355822
Change-Id: Id8cbb34f70214327e1f5cc96559e4ea66dc17889
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2801154
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>