[SystemZ] Optimize selects between 0 and -1

Since z has no setcc instruction as such, the choice of setBooleanContents
is a bit arbitrary.  Currently it's set to ZeroOrOneBooleanContent,
so we produced a branch-free form when selecting between 0 and 1,
but not when selecting between 0 and -1.  This patch handles the latter
case too.

At some point I'd like to measure whether it's better to use conditional
moves for constant selects on z196, but that's future work.