Interpreter: Fix code-gen for postfix ++/-- used only for side effect

Add a test case that previously asserted (due to incorrect code-gen
underflowing the stack).

Change-Id: I9df7a08b9ac5c7b5bc246129f3383dc723173351
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/228351
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed