[X86] Avoid passing _mm_undefined* to builtin_shufflevector if we are able to...

[X86] Avoid passing _mm_undefined* to builtin_shufflevector if we are able to pass the first input a second time.

This is more consistent with other usages of builtin_shufflevector. Later optimization passes or codegen will detect the duplicate vector and replace it with undef. Using _mm_undefined just puts a zeroinitializer that still needs to be optimized out later.

llvm-svn: 333944