LoopVectorize: Move call of canHoistAllLoads to canVectorizeWithIfConvert

We only want to check this once, not for every conditional block in the loop.

No functionality change (except that we don't perform a check redudantly
anymore).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181942 91177308-0d34-0410-b5e6-96231b3b80d8