AArch64: avoid deleting the current iterator in a loop.

std::map invalidates the iterator to any element that gets deleted, which means
we can't increment it correctly afterwards. This was causing Darwin test
failures.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215233 91177308-0d34-0410-b5e6-96231b3b80d8