Reformat some bits of AllocaPromoter and simplify the name and type of
our visiting datastructures in the AllocaPromoter/SSAUpdater path of
SROA. Also shift the order if clears around to be more consistent.

No functionality changed here, this is just a cleanup.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188144 91177308-0d34-0410-b5e6-96231b3b80d8