Add the unconditional branch instruction, improve diagnostics for block
references.

PiperOrigin-RevId: 201872745
diff --git a/lib/Parser/Token.h b/lib/Parser/Token.h
index c8f856c..dde0722 100644
--- a/lib/Parser/Token.h
+++ b/lib/Parser/Token.h
@@ -51,6 +51,7 @@
 
     // Keywords.
     kw_bf16,
+    kw_br,
     kw_cfgfunc,
     kw_extfunc,
     kw_f16,