[InstCombine] form uaddsat from add+umin (PR14613)

This is the last step towards solving the examples shown in:
https://bugs.llvm.org/show_bug.cgi?id=14613

With this change, x86 should end up with psubus instructions
when those are available.

All known codegen issues with expanding the saturating intrinsics
were resolved with:
D59006 / rL356855

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.

llvm-svn: 357012