Remove Compiler::toBackend calls.

We can now use the un-entangled SkSL::ToBackend() calls instead.

Bug: b/317363889
Bug: b/40045064
Change-Id: I50f52f735000eafbdfdc718e98497dc4c1c3c713
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/793578
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
4 files changed