Refactor get_canonical_form_slice so that it uses as_index_literal

fd4c5d91 · Brandon T. Willard · Brandon T. Willard · 0cbf8557 · fd4c5d91 · fd4c5d91
--- a/aesara/tensor/subtensor.py
+++ b/aesara/tensor/subtensor.py
--- a/tests/tensor/test_subtensor.py
+++ b/tests/tensor/test_subtensor.py