[LV] Remove implicit single basic block assumption

This patch is part of D28975's breakdown - no change in output intended.

LV's code currently assumes the vectorized loop is a single basic block up
until predicateInstructions() is called. This patch removes two manifestations
of this assumption (loop phi incoming values, dominator tree update) by
replacing the use of vectorLoopBody with the vectorized loop's latch/header.

Differential Revision: https://reviews.llvm.org/D32040

llvm-svn: 300310