Replace clear_write/clone_with_ref_kind with Analysis::UpdateRefKind.

It turns out that walking an expression to update the refKind of its
embedded VariableReference is so useful that we had implemented it
twice. I will need to do it a third time in a followup CL.

Change-Id: Idd2794ea50f8c71725bb998ac8a625b36d076746
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/368118
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
4 files changed