[runtimeunroll] Support multiple exits to latch exit w/prolog loop

This patch extends the runtime unrolling infrastructure to support unrolling a loop with multiple exiting blocks branching to the same exit block used by the latch. It intentionally does not include a cost model change to enable this functionality unless appropriate force flags are used.

This is the prolog companion to D107381. Since this was LGTMed, a problem with DT updating was reported against that patch.  I roled in the analogous fix here as it seemed obvious, and not worth re-review.

As an aside, our prolog form leaves a lot of potential value on the floor when there is an invariant load or invariant condition in the loop being runtime unrolled. We should probably consider a "required prolog" heuristic.  (Alternatively, maybe we should be peeling these cases more aggressively?)

Differential Revision: https://reviews.llvm.org/D108262

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