Improve generation of not equals check

[ElektroKill]

Link to issue(s) this covers:
fixes #3423

Problem

The decompiler would use 0 for the != 0 check even if the type was an enum.

Solution

• Adjust the code to use ExpressionBuilder.ConvertConstantValue to generate the expression for the 0 constant
• A pretty test was added to cover the issue

[siegfriedpammer]

Thank you very much for your contribution!