[CostModel] Update default cost model for sadd/ssub overflow to match TargetLowering

The expansion for these was updated in https://reviews.llvm.org/D47927 but the cost model was not adjusted.

I believe the cost model was also incorrect for the old expansion.
The expansion prior to D47927 used 3 icmps using LHS, RHS, and Result
to calculate theirs signs. Then 2 icmps to compare the signs. Followed
by an And. The previous cost model was using 3 icmps and 2 selects.
Digging back through git blame, those 2 selects in the cost model used to
be 2 icmps, but were changed in https://reviews.llvm.org/D90681

Differential Revision: https://reviews.llvm.org/D110739