Rename the VOID type to INVALID and clean up obsolete comments.

The VOID type isn't used for anything resembling what "void" means in C,
so rename it to INVALID to avoid confusion.

lib/codegen/src/ir/builder.rs

@@ -28,7 +28,7 @@ pub trait InstBuilderBase<'f>: Sized {
     /// Insert an instruction and return a reference to it, consuming the builder.
     ///
     /// The result types may depend on a controlling type variable. For non-polymorphic
-    /// instructions with multiple results, pass `VOID` for the `ctrl_typevar` argument.
+    /// instructions with multiple results, pass `INVALID` for the `ctrl_typevar` argument.
     fn build(self, data: InstructionData, ctrl_typevar: Type) -> (Inst, &'f mut DataFlowGraph);
 }