Cleanup the way we test niche values

[danielsn]

Description of changes:

The current code for testing niche values is complicated and hard to understand.
Rewrite it so it more closely follows the pattern in rustc.
This allows removal of the "Pointer" special case.
It also removes verbiage in the output goto that was making debugging more annoying.

Resolved issues:

Resolves #1540

Call-outs:

• is_zero works on pointers. Remove the assert that prevented it from doing so.

Testing:

• How is this change tested? Existing unit tests that cover niche optimization

• Is this a refactor change? Kinda?

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.

[tedinski]

lgtm, good links to ssa/cranelift!