[MemCpyOpt] Fix infinite loop in memset+memcpy fold (#98638)

For the case where the memcpy size is zero, this transform is a complex
no-op. This can lead to an infinite loop when the size is zero in a way
that BasicAA understands, because it can still understand that dst and
dst + src_size are MustAlias.

I've tried to mitigate this before using the isZeroSize() check, but we
can hit cases where InstSimplify doesn't understand that the size is
zero, but BasicAA does.

As such, this bites the bullet and adds an explicit isKnownNonZero()
check to guard against no-op transforms.

Fixes https://github.com/llvm/llvm-project/issues/98610.

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