Implement WGSL index- and field-access lvalues.

This restores several of the tests that were broken in the previous
CL. We don't yet support lvalues for swizzles, or indexes into
vectors, since those can't be represented as a PointerLValue.

Change-Id: I78927145146a8ca267c0a2f0386a56941ec82faa
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/706378
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
9 files changed