Fix SearchValues<string>.Contains issue if one value is a prefix of all the others

[MihaZupan]

We remove values that are suffixes of other values when constructing a SearchValues<string> instance.
If we're left with just one value, we'll use the single-value implementation, but we weren't accounting for values that may have been removed, and ContainsCore would return a false negative for such values.

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Issue Details

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We remove values that are suffixes of other values when constructing a SearchValues<string> instance.
If we're left with just one value, we'll use the single-value implementation, but we weren't accounting for values that may have been removed, and ContainsCore would return a false negative for such values.

Author:	MihaZupan
Assignees:	MihaZupan
Labels:	area-System.Buffers
Milestone:	9.0.0