SDAG: Handle scalarizing an extend of a <1 x iN> vector.

Just scalarize the element and rebuild a vector of the result type
from that.

rdar://13281568

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176614 91177308-0d34-0410-b5e6-96231b3b80d8