LoopVectorizer: Don't attempt to vectorize extractelement instructions

The loop vectorizer does not currently understand how to vectorize
extractelement instructions. The existing check, which excluded all
vector-valued instructions, did not catch extractelement instructions because
it checked only the return value. As a result, vectorization would proceed,
producing illegal instructions like this:

  %58 = extractelement <2 x i32> %15, i32 0
  %59 = extractelement i32 %58, i32 0

where the second extractelement is illegal because its first operand is not a vector.