Re-add flag to force opList splitting reduction

We need this again because I had to turn off opList splitting reduction by default but would like to keep testing it on some bots

Change-Id: I9e38d5a3d823e86799055892d191a5643a4b98ac
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/212193
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
2 files changed