Simplify SplitVecRes_EXTRACT_SUBVECTOR. This means

that it no longer handles non-power-of-two vectors.
However it previously only handled them sometimes,
depending on obscure numerical relationships between
the index and vector type.  For example, for a vector
of length 6, it would succeed if and only if the
index was an even multiple of 6.  I consider this
more confusing than useful.

llvm-svn: 59122