Fix incorrect CoercedConstantValue<ssize_t> usages

[EgorBo]

On 32-bit ssize_t is basically int32_t and CoercedConstantValue<ssize_t> may just truncate a TYP_LONG-typed VN constant. One obvious buggy behavior is in IsVNIntegralConstant where it may lead to any TYP_LONG to be reported as "fits into int32".

So IsVNIntegralConstant should use int64_t instead.

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