[CGP] adjust target constraints for forming uaddo

There are 2 changes visible here:
1. There's no reason to limit this transform based on number
   of condition registers. That diff allows PPC to produce 
   slightly better (dot-instructions should be generally good) 
   code.
   Note: someone that cares about PPC codegen might want to 
   look closer at that output because it seems like we could
   still improve this.

2. We (probably?) should not bother trying to form uaddo (or
   other overflow ops) when there's no target support for such
   an op. This goes beyond checking whether the op is expanded
   because both PPC and AArch64 show better codegen for standard
   types regardless of whether the op is legal/custom.

llvm-svn: 353001