Fix shl folding in DAG combiner.

Summary: The code responsible for shl folding in the DAGCombiner was assuming incorrectly that all constants are less than 64 bits. This patch simply changes the way values are compared.

Test Plan: A regression test included.

Reviewers: andreadb

Reviewed By: andreadb

Subscribers: andreadb, test, llvm-commits

Differential Revision: http://reviews.llvm.org/D10602

llvm-svn: 240291