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

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

When range metadata is provided, it should be used to constant fold comparisons with constant values.

Reviewers: sanjoy, hfinkel

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D12988

llvm-svn: 248402