This CL adds tests for the if statement AST node. Bug: tint:11 Change-Id: Ice3f281e1dfd72d76f43767f7b3040af862f4a58 Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/16540 Reviewed-by: Sarah Mashayekhi <sarahmashay@google.com>