GL: Fix the RewriteRepeatedAssignToSwizzled workaround This transformation splits chained assignments where the LHS is assigning to a swizzle. However, instead of retaining the assignment operator (like *=, += etc), it was accidentally unconditionally using `=`. Bug: chromium:1394345 Change-Id: Icaef159e1fb57e720e08d4b4f6603f08d9cd2bfe Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4381648 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Yuxin Hu <yuxinhu@google.com>