[llvm] [DA] Rewrite BanerjeeMIV test with safe APInt interval arithmetic (PR #207662)

[Ruoyu Qiu via llvm-commits]

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cabbaken wrote:

Documented this in CoefficientInfo. It is null when the subscript does not use that loop level or when ScalarEvolution cannot compute an exact backedge count.
A missing count with a nonzero coefficient is treated as unbounded; a zero coefficient still produces the interval zero.

https://github.com/llvm/llvm-project/pull/207662