folding policies with IRBuilder. The default,
provided by ConstantFolder, is to do minimal
folding like now: what ConstantExpr provides.
An alternative is to use TargetFolder, which
uses target information to fold constants more.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@54640 91177308-0d34-0410-b5e6-96231b3b80d8
