[InstCombine] Demand bits of UMin

This is the umin alternative to the umax code from rL344237. We use
DeMorgans law on the umax case to bring us to the same thing on umin,
but using countLeadingOnes, not countLeadingZeros.

Differential Revision: https://reviews.llvm.org/D53036

llvm-svn: 344239