Optimize sext <4 x i8> and <4 x i16> to <4 x i64>.
Patch by Ahmad, Muhammad T <muhammad.t.ahmad@intel.com>



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177421 91177308-0d34-0410-b5e6-96231b3b80d8