Added dead variable / code elimination to skslc.

BUG=skia:

Change-Id: Ib037730803a8f222f099de0e001fe06ad452a22c
Reviewed-on: https://skia-review.googlesource.com/7584
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Ben Wagner <benjaminwagner@google.com>
37 files changed