S_uiv_2buf: code quality improvements suggested in GH#22927

Author: richardleach

Diff for: sv_inline.h

@@ -1055,14 +1055,16 @@ S_uiv_2buf(char *const buf, const IV iv, UV uv, const int is_uv, char **const pe
     word_ptr = (U16*)ptr;
     word_table = (U16*)Perl_int2str_table.arr;
 
-    if (UNLIKELY(is_uv))
+    if (is_uv)
         sign = 0;
     else if (iv >= 0) {
         uv = iv;
         sign = 0;
     } else {
-        /* Using 0- here to silence bogus warning from MS VC */
-        uv = (UV) (0 - (UV) iv);
+        /* This is NEGATE_2UV(iv), which can be found in handy.h. */
+        /* sv_inline.h does not include handy.h because the latter
+         * would then get included twice into .c files. */
+        uv = (ASSUME((iv) < 0), (UV)-((iv) + 1) + 1U);
         sign = 1;
     }