Vulkan: Don't invalidate index buffers on vertex change. The vertex state should not affect index state changes. Split off from a larger CL. Bug: angleproject:4622 Change-Id: Ie7c3148a16b5da89b79d95e1de130a1ff0e6c634 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2233404 Reviewed-by: Tim Van Patten <timvp@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>