[InstCombine] fold sub-of-umax to 0-usubsat

Op0 - umax(X, Op0) --> 0 - usub.sat(X, Op1)

I'm not sure if this is really an improvement in IR because
we probably have better recognition/analysis for min/max,
but this lines up with the fold we do for the icmp+select
idiom and removes another diff from D98152.

This is similar to the previous fold in the code that was
added with:
83c2fb9f
baa6a851

https://alive2.llvm.org/ce/z/5MrVB9