Remove code that prevented lowering shuffles if they are used by load and...

Remove code that prevented lowering shuffles if they are used by load and themselves used by a extract_vector_elt. This was done to allow the DAG combiner to collapse to a single element load. Unfortunately, sometimes the extract_vector_elt would disappear before DAG combine could do the transformation leaving a vector_shuffle that isel couldn't handle. New code lets the shuffle be converted to a target specific node, but then adds a combine routine that can convert target specific nodes back to vector_shuffles if the folding criteria are met.

llvm-svn: 153080