Fix overflow in matrix-multiply constant folding.

Previously, this would generate invalid code:
http://review.skia.org/726579

Change-Id: I697e51b6a0aa670f3986fc95bf01aaa49c57e1a3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/726578
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
8 files changed