Casts to `void` did not have a semantic conversion type in the AST, so they also weren't getting generated correctly in the IR. I've added a `VoidConversion` class to the AST, along with tests. I've also added IR translation for such conversions, using a new `ConvertToVoid` opcode. I'm not sure if it's really necessary to generate an instruction to represent this, but it may be useful for detecting values that are explicitly unused (e.g. return value from a call).
I added two new sanity queries for the IR to detect the following:
- IR blocks with no successors, which usually indicates bad IR translation
- Phi instruction without an operand for one of the predecessor blocks.
These sanity queries found another subtle IR translation bug. If an expression that is normally translated as a condition (e.g. `&&`, `||`, or parens in certain contexts) has a constant value, we were not creating a `TranslatedExpr` for the expression at all. I changed it to always treat a constant condition as a non-condition expression.
