Fix the build with MSVC 2013 after new shuffle code

MSVC gives this awesome diagnostic:

..\lib\Target\X86\X86ISelLowering.cpp(7085) : error C2971: 'llvm::VariadicFunction1' : template parameter 'Func' : 'isShuffleEquivalentImpl' : a local variable cannot be used as a non-type argument
        ..\include\llvm/ADT/VariadicFunction.h(153) : see declaration of 'llvm::VariadicFunction1'
        ..\lib\Target\X86\X86ISelLowering.cpp(7061) : see declaration of 'isShuffleEquivalentImpl'

Using an anonymous namespace makes the problem go away.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215744 91177308-0d34-0410-b5e6-96231b3b80d8