[SCEV] Support SCEVUMinExpr in getRangeRef.

This patch adds support for SCEVUMinExpr to getRangeRef,
similar to the support for SCEVUMaxExpr.

Reviewers: sanjoy.google, efriedma, reames, nikic

Reviewed By: sanjoy.google

Differential Revision: https://reviews.llvm.org/D67177

llvm-svn: 371768