stage2: optional comparison and 0-bit payloads

* Sema: implement peer type resolution for optionals and null.
 * Rename `Module.optionalType` to `Type.optional`.
 * LLVM backend: re-use anonymous values. This is especially useful when
   isByRef()=true because it means re-using the same generated LLVM globals.
 * LLVM backend: rework the implementation of is_null and is_non_null
   AIR instructions. Generate slightly better LLVM code, and also fix
   the behavior for optionals whose payload type is 0-bit.
 * LLVM backend: improve `cmp` AIR instruction lowering to support
   pointer-like optionals.
 * `Value`: implement support for equality-checking optionals.

src/Module.zig

@@ -4249,20 +4249,6 @@ pub fn errNoteNonLazy(
     };
 }
 
-pub fn optionalType(arena: *Allocator, child_type: Type) Allocator.Error!Type {
-    switch (child_type.tag()) {
-        .single_const_pointer => return Type.Tag.optional_single_const_pointer.create(
-            arena,
-            child_type.elemType(),
-        ),
-        .single_mut_pointer => return Type.Tag.optional_single_mut_pointer.create(
-            arena,
-            child_type.elemType(),
-        ),
-        else => return Type.Tag.optional.create(arena, child_type),
-    }
-}
-
 pub fn errorUnionType(
     arena: *Allocator,
     error_set: Type,