Omit dead SkSL functions

Now that SkSL inlines functions, dead functions are very common. This
change causes them to be omitted from the final output.

Change-Id: Ie466a3f748812eff1a368498365c89d73ab0b7be
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/292684
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
9 files changed