[LegalizeVectorTypes] Have SplitVecOp_TruncateHelper fall back to...

[LegalizeVectorTypes] Have SplitVecOp_TruncateHelper fall back to SplitVecOp_UnaryOp if splitting the output type would be a legal type.

SplitVecOp_TruncateHelper tries to introduce a multilevel truncate to avoid scalarization. But if splitting the result type would still be a legal type we don't need to do that.

The comment block at the top of the function implied that this was already implemented. I looked back through the history and it doesn't look to have ever been checked.

llvm-svn: 347479