[ConstantFolding] add simplifications for funnel shift intrinsics

This is another step towards being able to canonicalize to the funnel shift 
intrinsics in IR (see D49242 for the initial patch). 
We should not have any loss of simplification power in IR between these and 
the equivalent IR constructs.

Differential Revision: https://reviews.llvm.org/D50848

llvm-svn: 340022