Revert "Add a target legalize hook for SplitVectorOperand"

This reverts commit 187198. It broke the bots.

The soft float test probably needs a -triple because of name differences.
On the hard float test I am getting a "roundss $1, %xmm0, %xmm0", instead of
"vroundss $1, %xmm0, %xmm0, %xmm0".

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187201 91177308-0d34-0410-b5e6-96231b3b80d8