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.

test/behavior/optional.zig

@@ -44,3 +44,32 @@ test "optional pointer to size zero struct" {
     var o: ?*EmptyStruct = &e;
     try expect(o != null);
 }
+
+test "equality compare optional pointers" {
+    try testNullPtrsEql();
+    comptime try testNullPtrsEql();
+}
+
+fn testNullPtrsEql() !void {
+    var number: i32 = 1234;
+
+    var x: ?*i32 = null;
+    var y: ?*i32 = null;
+    try expect(x == y);
+    y = &number;
+    try expect(x != y);
+    try expect(x != &number);
+    try expect(&number != x);
+    x = &number;
+    try expect(x == y);
+    try expect(x == &number);
+    try expect(&number == x);
+}
+
+test "optional with void type" {
+    const Foo = struct {
+        x: ?void,
+    };
+    var x = Foo{ .x = null };
+    try expect(x.x == null);
+}