[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.

llvm-svn: 196578