This is part of of D52508. It uses the demand bits of umax(A, C) to prove we use just A, so long as:
"The lowest non-zero bit of DemandMask is higher than the highest non-zero bit of C"
Conceptually, this works because:
if A > C, we pick A. No big deal.
if A < C then all the demanded bits of A == demanded bits of C, so we can pick either and get the same result. So we pick A.