[SCEV] Simplify umin/max of zext and sext of the same value

This is a common idiom which arises after induction variables are widened, and we have two or more exit conditions.  Interestingly, we don't have instcombine or instsimplify support for this either.

Differential Revision: https://reviews.llvm.org/D69006

llvm-svn: 375349