[InstCombine] form uaddsat from add+umin (PR14613)
This is the last step towards solving the examples shown in:
With this change, x86 should end up with psubus instructions
when those are available.
We also have some early evidence in D58872 that using the intrinsics
will lead to better perf. If some target regresses from this, custom
lowering of the intrinsics (as in the above for x86) may be needed.