Add SPIR-V support for modf intrinsic.

Change-Id: I887e700a7bf11bf2d5359c9721798f72f00e53f3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/342756
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
6 files changed