Set overflow flag for add/sub hi on x86 TYP_LONG

Set overflow flag for add/sub hi on x86 TYP_LONG

When we create the hi operation for add and sub on TYP_LONG, we don't
carry the overflow flag to the hi operation. This change sets the overflow
flag on hiResult in lower if it was set on loResult, and adds GT_ADD_HI
and GT_SUB_HI to the operations that can have overflow. We also need to
pass the unsigned flag to the high part in the instance that we are
dealing with an add or subtract with overflow. Fixes #4596.