assemble.c: do not warn on valid SBYTE optimizations

Do not warn on valid SBYTE optimizations.  If we are optimizing and
match one of the SBYTE conditions, do not error out.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
diff --git a/doc/changes.src b/doc/changes.src
index c0eeddc..352bbd6 100644
--- a/doc/changes.src
+++ b/doc/changes.src
@@ -25,6 +25,9 @@
 
 \b Fix \c{%include} inside multi-line macros or loops.
 
+\b Fix error where NASM would generate a spurious warning on valid
+  optimizations of immediate values.
+
 
 \S{cl-2.04} Version 2.04