[DAGCombiner] Fix & simplify constant folding of sext/zext.

Summary: This patch fixes the cases of sext/zext constant folding in DAG combiner where constans do not fit 64 bits. The fix simply removes un$

Test Plan: New regression test included.

Reviewers: RKSimon

Reviewed By: RKSimon

Subscribers: RKSimon, llvm-commits

Differential Revision: http://reviews.llvm.org/D10607

llvm-svn: 240991