Vulkan: Fix `vec4(...).zxwy[index]` The code was actually correct, but included an unnecessary assertion. The code incorrectly assumed `.zxwy[index]` is pre-processed with a lookup table and turned into `[index']`, but it wasn't if the LHS is a constructor. The change just removes the assertion because it was correctly handling that already. Bug: angleproject:40096715 Change-Id: Ib1365f95a255ce3654831af55429361d35c026c4 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5949527 Reviewed-by: Yuxin Hu <yuxinhu@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>