Migrate Analysis::IsTrivialExpression to its own cpp.

In a followup, I'll be making changes to this algorithm for skia:13378.
This CL migrates the code as-is.

Change-Id: I86cfddeb7c8eb6d4e191371944fb7c3707f13ba6
Bug: skia:13378
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/546418
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Arman Uguray <armansito@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
5 files changed