RFR: 8320649: C2: Optimize scoped values [v4]

[Roland Westrelin]

On Wed, 17 Jan 2024 11:10:08 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

> Why can this not be done in the first traversal, and why does this (down) traversal do the right thing?

The first traversal starts from the end of the method and follows control paths until it reaches the `Thread.scopedValueCache()` call. Given the shape of the method and that some paths may have been trimmed and end with an uncommon trap, it could reach either the first or the second if that probe the cache first.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16966#discussion_r1466635670