[SelectionDAG] Teach the vector scalarizer about TRUNCATE.

When a truncate node defines a legal vector type but uses an illegal
vector type, the legalization process was splitting the vector until
<1 x vector> type, but then it was failing to scalarize the node because
it did not know how to handle TRUNCATE.

<rdar://problem/14989896>

llvm-svn: 190830