Improve gencode for inlined blend and composition functions.

Using `return` instead of assigning to the output color removes an
unnecessary temporary variable from the output.

Change-Id: Ica31e290f8745a7309ae32c7148516d2189308ea
Bug: skia:10549
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/328386
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
2 files changed