Clean up the logic of vector shuffles -> vector shifts.

Clean up the logic of vector shuffles -> vector shifts.
Also teach this logic how to handle target specific shuffles if
needed, this is necessary while searching recursively for zeroed
scalar elements in vector shuffle operands.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@112348 91177308-0d34-0410-b5e6-96231b3b80d8