[InstSimplify] fold min/max with matching min/max operands

I think this is the last remaining translation of an existing
instcombine transform for the corresponding cmp+sel idiom.

This interpretation is more general though - we can remove
mismatched signed/unsigned combinations in addition to the
more obvious cases.

min/max(X, Y) must produce X or Y as the result, so this is
just another clause in the existing transform that was already
matching a min/max of min/max.