[VPlan] Add SCEV support for abs intrinsic (#195678)
Teach `getSCEVExprForVPValue` to model `llvm.abs` via
`ScalarEvolution::getAbsExpr`, preserving the intrinsic's
is_int_min_poison flag as the SCEV IsNSW argument. Add a unit test
covering both poison and wrapping llvm.abs forms.
