Add a target legalize hook for SplitVectorOperand (again)

CustomLowerNode was not being called during SplitVectorOperand,
meaning custom legalization could not be used by targets.

This also adds a test case for NVPTX that depends on this custom
legalization.

Differential Revision: http://llvm-reviews.chandlerc.com/D1195

Attempt to fix the buildbots by making the X86 test I just added platform independent