[DebugInfo][RemoveDIs] Have LICM insert at iterator positions (#73671)

Because we're storing some extra debug-info information in the iterator
class, we need to insert new LICM-created stores using such iterators.
Switch LICM to storing iterators instead of pointers when it promotes
variables in loops, add a test for the desired behaviour, and enable
RemoveDIs instrumentation on a variety of other LICM tests for good
measure.

(This would appear to be the only pass in LLVM that needs to store
iterators on the heap).