AArch64: Re-enable AArch64AddressTypePromotion

This reverts commits r212189 and r212190.

While this pass was accidentally disabled (until r212073), r205437
slipped in a use of `auto` that should have been `auto&`.

This fixes PR20188.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212201 91177308-0d34-0410-b5e6-96231b3b80d8