Vulkan: Fix assertion in xfb of indirect calls An assertion was fired which was only valid when emulating transform feedback. The relevant block is conditioned to the respective feature. Bug: angleproject:3571 Change-Id: I287ef6d94c920ccda742e4032bfc389409a38e1f Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2599951 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>