[DAGCombiner][X86][SystemZ] Canonicalize SSUBO with immediate RHS to SADDO by...

[DAGCombiner][X86][SystemZ] Canonicalize SSUBO with immediate RHS to SADDO by negating the immediate.

This lines up with what we do for regular subtract and it matches up better with X86 assumptions in isel patterns that add with immediate is more canonical than sub with immediate.

Differential Revision: https://reviews.llvm.org/D60020

llvm-svn: 358027