Speed up program visitors in the Inliner.

They don't inspect Expressions at all, but by default, a ProgramVisitor
will completely traverse every Expression. This change improved local
runs of nanobench on sksl_large by ~7%.

Change-Id: I7604ffa220c6081c3cd39b5a22520fb41421fcba
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/385878
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
1 file changed