Small optimizations in range checks

Checks of the form '1 <= x && x <= M' were rewritten in the form
'(unsigned)x - 1 < (unsigned)M', which is usually more efficient.
(Other similar checks have similar translations.) Although
some compilers do these optimizations, that does not happen
for all compilers or all cases.

testes/utf8.lua

@@ -123,6 +123,9 @@ checkerror("continuation byte", utf8.offset, "𦧺", 1, 2)
 checkerror("continuation byte", utf8.offset, "𦧺", 1, 2)
 checkerror("continuation byte", utf8.offset, "\x80", 1)
 
+-- error in indices for len
+checkerror("out of string", utf8.len, "abc", 0, 2)
+checkerror("out of string", utf8.len, "abc", 1, 4)
 
 
 local s = "hello World"