Add a target legalize hook for SplitVectorOperand

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187198 91177308-0d34-0410-b5e6-96231b3b80d8