[SystemZ] Fix addcarry of usubo (PR42512)

Only custom lower uaddo+addcarry or usubo+subcarry chains and leave
mixtures like usubo+addcarry or uaddo+subcarry to the generic
legalizer. Otherwise we run into issues because SystemZ uses
different CC values for carries and borrows.

Fixes https://bugs.llvm.org/show_bug.cgi?id=42512.

Differential Revision: https://reviews.llvm.org/D64213

llvm-svn: 365242