Migrate IRGenerator::setRefKind to Analysis::MakeAssignmentExpr.

Fundamentally this function wasn't a generic "set the refKind on an
expression" at all; it was intended specifically as "this expression
must be IsAssignable and contain a VariableReference." Also, it needs
nothing at all from the IRGenerator. Moving it out is important because
it will be used by Make functions for things like VariableReference.

Change-Id: I52d613989cd5fb0feb2e98ae454943cdacb9404b
Bug: skia:11342
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/376841
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
3 files changed