[LoopVectorizer] LoopVectorBody doesn't need to be a vector. NFC.

LoopVectorBody was changed from a single pointer to a SmallVector when
store predication was introduced in r200270. Since r247139, store predication
no longer splits the vector loop body in-place, so we can go back to having
a single LoopVectorBody block.

This reverts the no-longer-needed changes from r200270.

llvm-svn: 269321