[Gm2] negate whole number overflow detection implemented

[Gaius Mulley]

Hello,

a small note to say that runtime checking of whole number overflow is
now implemented for the negate operator.  This works on subranges as
well as integers.  Currently committed on the master.  A simple obvious
example follows:

MODULE subrange ;

FROM libc IMPORT exit ;

VAR
   i: [-4..3] ;
BEGIN
   i := -4 ;
   i := -i ;
   exit(0)    (* should not get here if -fsoft-check-all is used *)
END subrange.

regards,
Gaius