Analyze reaching PHI in GetRangeFromAssertions (more branch foldings)

[EgorBo]

Some interesting diffs are produced.

I slowly mimic what SSA-based GetRange does, but purely on top of VNs and assertions. This should be, in theory, more powerful, doesn't require a separate "does overflow" path, can be called from Global Assertion Prop general optimizations.

simple example:

void Test(bool cond, int[] arr)
{
    int x = cond ? -1 : -2;   // [-2..-1]
    int y = arr?.Length ?? 0; // [0..Array.MaxLength]

    if (x > y) // always false -- it's now folded
        Console.WriteLine("below zero");
}

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