[LAA] Relax restrictions on early exits in loop structure

his is a preparation patch for supporting multiple exits in the loop vectorizer, by itself it should be mostly NFC. This patch moves the loop structure checks from LAA to their respective consumers (where duplicates don't already exist).  Moving the checks does end up changing some of the optimization warnings and debug output slightly, but nothing that appears to be a regression.

Why do this? Well, after auditing the code, I can't actually find anything in LAA itself which relies on having all instructions within a loop execute an equal number of times. This patch simply makes this explicit so that if one consumer - say LV in the near future (hopefully) - wants to handle a broader class of loops, it can do so.

Differential Revision: https://reviews.llvm.org/D92066

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