[InstCombine] fix sub(add(X,Y),umin(Y,Z)) --> add(X,usub.sat(Y,Z))

This patch fix bug left in D124503. We should do
sub(add(X,Z),umin(Y,Z)) --> add(X,usub.sat(Z,Y)) instead of
sub(add(X,Z),umin(Y,Z)) --> add(X,usub.sat(Y,Z)).

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D125352