[DAGTypeLegalizer] Handle widening truncate to vector of i1.

Previously, PromoteIntRes_TRUNCATE() did not handle the case where
the operand needs widening, which resulted in llvm_unreachable().

This patch adds the needed handling, along with a test case.

Review: Eli Friedman, Simon Pilgrim.
https://reviews.llvm.org/D31077

llvm-svn: 298357