HIR typeck cleanup: clarify and re-style `check_expr_unop` Two small cleanups: - The `oprnd_t` variable was previously used both for the type of unary operators' operands and for the type of the operator expression as a whole. I found this confusing, so I've changed it to only be used for the operand type. This also allowed for removing some negations from `if` conditions. - I did a bit of re-styling in the second commit to remove some indentation levels, which also means fewer things have to be spread across multiple lines. This part is more to-taste; I'd be fine dropping it if it doesn't actually help.