RFR: 8253037: G1: Improve check for string dedup

[Albert Mingkun Yang]

On Sun, 1 Nov 2020 11:38:56 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> That seems redundant, since this function is in StringDedup.

OK, then the fact that its neighbor, `enqueue_from_mark`, doesn't have such precondition (must be `java_string`), but shares the same naming schema could be a bit surprising. Or, maybe it's better to add the same precondition for `enqueue_from_mark` as well?

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PR: https://git.openjdk.java.net/jdk/pull/981