Migrate IsConstantExpression out of SkSLAnalysis.

Change-Id: I08717dd9e9ad20a18a623e06f0c66a0f697effa4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/456796
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
4 files changed