Store loop-unroll information inside ForStatement.

This will be useful when trying to determine the flattened size of a
program, and it is expensive to compute on demand.

Change-Id: I232d9189511502d4783e5542a9bfe0dff8ea8c4a
Bug: skia:12396
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/443883
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
7 files changed