[SCEV] Use umin_seq for BECount of multi-exit loops

When computing the BECount for multi-exit loops, we need to combine
individual exit counts using umin_seq rather than umin. This is
because an earlier exit may exit on the first iteration, in which
case later exit expressions will not be evaluated and could be
poisonous. We cannot propagate potential poison values from later
exits.

In particular, this avoids the introduction of "branch on poison"
UB when optimizing multi-exit loops.

Differential Revision: https://reviews.llvm.org/D124910

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