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.

1 files changed, 14 insertions, 0 deletions

diff --git a/src/type.zig b/src/type.zig
index a37a3bfc03..278b8af18d 100644
--- a/src/type.zig
+++ b/src/type.zig
@@ -4031,6 +4031,20 @@ pub const Type = extern union {
         });
     }
 
+    pub fn optional(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 smallestUnsignedBits(max: u64) u16 {
         if (max == 0) return 0;
         const base = std.math.log2(max);