Reduce self-assignment expressions (`x = x`) into just `x`.

Change-Id: I44356aa781f208f4941dbeb617a30318e36c1081
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/367062
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
18 files changed