Declare unreachable intrinsic as unreachable

[adpaco]

Description of changes:

Includes the case for unreachable in the intrinsics match that is expected to be unreachable. This case was removed in #1100 (includes a test for unreachable), but as suggested by @zhassan-aws this way we avoid confusion.

Resolved issues:

Resolves #1115

Call-outs:

Testing:

• How is this change tested? Existing regression.

• Is this a refactor change? No.

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[zhassan-aws]

Thanks!