Split IRGenerator's VarDeclaration processing into two parts

This allows things to align better between the IR and DSL sides
and gives us equivalent error handling on both sides.

Change-Id: I6d5569e29df51a4d1a6cb0ad1e6611d419dfe30c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/373737
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Auto-Submit: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
8 files changed