[X86] Simplify code. NFC.

Replaced some logic that checked if a build_vector node is doing a splat of a
non-undef value with a call to method BuildVectorSDNode::getSplatValue().
No functional change intended.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223354 91177308-0d34-0410-b5e6-96231b3b80d8