[LV] Bail out early if BTC+1 wraps.

Currently we fail to detect the case where BTC + 1 wraps, i.e. the
vector trip count is 0, In those cases, the minimum iteration count
check will fail, and the ve

[LV] Bail out early if BTC+1 wraps.

Currently we fail to detect the case where BTC + 1 wraps, i.e. the
vector trip count is 0, In those cases, the minimum iteration count
check will fail, and the vector code will never be executed.

Explicitly check for this condition in computeMaxVF and avoid trying to
vectorize alltogether.

Note that a number of tests needed to be updated, because the vector
loop would never be executed given the input IR.

Fixes https://github.com/llvm/llvm-project/issues/122558.

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