[Bug 24848] Use range metadata to constant fold comparisons between two values

Summary:
This is the second part of fixing bug 24848 https://llvm.org/bugs/show_bug.cgi?id=24848.

If both operands of a comparison have range metadata, they should be used to constant fold the comparison.

Reviewers: sanjoy, hfinkel

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D13177

llvm-svn: 248650