clean up sksl dead variable handling

Change-Id: I301e82bf87d976e59a02b0f383da67eaf5a8795a
Reviewed-on: https://skia-review.googlesource.com/18494
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
4 files changed