[CVP] @llvm.abs() handling

Iff we know the sigdness domain of the argument,
we can either skip @llvm.abs, or do negation directly.

Notably, INT_MIN can belong to either domain:
* X u<= INT_MIN --> X  is always fine
  https://alive2.llvm.org/ce/z/QB8j-C https://alive2.llvm.org/ce/z/7sFKpS
* X s<= 0 --> -X  is always fine
  https://alive2.llvm.org/ce/z/QbGSyq https://alive2.llvm.org/ce/z/APsN84

If all else fails, try to inferr NSW flag:
https://alive2.llvm.org/ce/z/qCJfYm