HPCC-12644 Fix problems in 32bit folding int64 return types

intresult was signed, causing the '+' to decrement the value, resulting
in a value 2^32 smaller than it should have been.

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
There's a comment on line 1276 that perhaps can now be removed? (or if not we should fix the other cases that are wrong).