Add Make factory functions to literal types.

This is mostly for consistency with other IRNode types; there isn't any
optimization opportunity or error checking for literals.

Change-Id: I0e6a55cddbd814585ecafeddab4e591fe5113911
Bug: skia:11342
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/383996
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
14 files changed