[Codegen] Add intrinsics 'absdiff' and corresponding SDNodes for absolute difference operation

This adds new intrinsics "*absdiff" for absolute difference ops to facilitate efficient code generation for "sum of absolute differences" operation.
The patch also contains the introduction of corresponding SDNodes and basic legalization support.Sanity of the generated code is tested on X86.

This is 1st of the three patches.

Patch by Shahid Asghar-ahmad!

llvm-svn: 242409