LoopVectorize: Hoist conditional loads if possible

InstCombine can be uncooperative to vectorization and sink loads into
conditional blocks. This prevents vectorization.

Undo this optimization if there are unconditional memory accesses to the same
addresses in the loop.

radar://13815763

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181860 91177308-0d34-0410-b5e6-96231b3b80d8