[LV] Add initial legality checks for early exit loops with side effects (#145663)
This adds initial support to LoopVectorizationLegality to analyze loops
with side effects (particularly stores to memory) and an uncountable
exit. This patch alone doesn't enable any new transformations, but
does give clearer reasons for rejecting vectorization for such a loop.
The intent is for a loop like the following to pass the specific checks,
and only be rejected at the end until the transformation code is
committed:
```
// Assume a is marked restrict
// Assume b is known to be large enough to access up to b[N-1]
for (int i = 0; i < N; ++) {
a[i]++;
if (b[i] > threshold)
break;
}
```
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