Clean up SkSL inliner and allow it to be disabled.

- When fInlineThreshold is zero, the inliner doesn't need to run at all.
- Inlining functionality outside of `analyze` can be made private now;
  it's not referenced by the IRGenerator any more.

Change-Id: If61fd8998bc024201bf4489b7aa48ac4d117e449
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/325617
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
3 files changed