[PM] Fix a bad invalid densemap iterator bug in the new invalidation

[PM] Fix a bad invalid densemap iterator bug in the new invalidation
logic.

Yup, the invalidation logic has an invalid iterator bug. Can't make this
stuff up.

We can recursively insert things into the map so we can't cache the
iterator into that map across those recursive calls. We did this
differently in two places. I have an end-to-end test that triggers at
least one of them. I'm going to work on a nice minimal test case that
triggers these, but I didn't want to leave the bug in the tree while
I tried to trigger it.

Also, the dense map iterator checking stuff we have now is awesome. =D

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@288135 91177308-0d34-0410-b5e6-96231b3b80d8