Refactored SkSL convertDo & convertFor

No functionality changes, but the new variants of the methods make them
more accessible from upcoming DSL code.

Change-Id: Iaa9f1fab31cb7db00007b00d7d3b88ff5b9696b6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/356103
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
2 files changed