stage2: implement union coercion to its own tag

 * AIR: add `get_union_tag` instruction
   - implement in LLVM backend
 * Sema: implement == and != for union and enum literal
   - Also implement coercion from union to its own tag type
 * Value: implement hashing for union values

The motivating example is this snippet:

    comptime assert(@typeInfo(T) == .Float);

This was the next blocker for stage2 building compiler-rt.
Now it is switch at compile-time on an integer.

1 files changed, 8 insertions, 0 deletions

diff --git a/src/type.zig b/src/type.zig
index bb798959f4..781fe74d45 100644
--- a/src/type.zig
+++ b/src/type.zig
@@ -2487,6 +2487,12 @@ pub const Type = extern union {
         };
     }
 
+    pub fn unionFieldType(ty: Type, enum_tag: Value) Type {
+        const union_obj = ty.cast(Payload.Union).?.data;
+        const index = union_obj.tag_ty.enumTagFieldIndex(enum_tag).?;
+        return union_obj.fields.values()[index].ty;
+    }
+
     /// Asserts that the type is an error union.
     pub fn errorUnionPayload(self: Type) Type {
         return switch (self.tag()) {
@@ -3801,6 +3807,8 @@ pub const Type = extern union {
         };
     };
 
+    pub const @"bool" = initTag(.bool);
+
     pub fn ptr(arena: *Allocator, d: Payload.Pointer.Data) !Type {
         assert(d.host_size == 0 or d.bit_offset < d.host_size * 8);