[DAG] ExpandIntRes_MINMAX - simplify cases with sufficient number of sign bits

When legalizing a smax/smin/umax/umin op, if we know that the upper half is all sign bits, then we can perform the op on the lower half and then sign extend the result to the upper half.

Alive2: https://alive2.llvm.org/ce/z/rk8Rfd

Fixes #58630