Add WGSL support for comma-expressions.

We can now use `assembleExpression` to cause side effects from the
left-hand side to occur, and then just discard the result.

Change-Id: I9e52110879e0916f7f9dce1257c62e04603aa3c1
Bug: skia:13092
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/706976
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Arman Uguray <armansito@google.com>
Commit-Queue: Arman Uguray <armansito@google.com>
7 files changed