Merges two StringBuilder.append(char) calls into a single append(char, char) call

[wenshao]

The purpose of this draft PR is to explore merging two consecutive StringBuilder.append(char) calls into append(char, char) in the C2 optimizer, reducing one ensureCapacity call and triggering more optimizations, such as MergeStore.

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Progress

• Change must not contain extraneous whitespace
• Commit message must refer to an issue
• Change must be properly reviewed

Error

 ⚠️ Could not determine the source for this merge. A Merge PR title must be specified in the format: ^Merge ([-/.\w:+]+)$ to allow verification of the merge contents.

Integration blocker

 ⚠️ Dependency #26911 must be integrated first

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/28228/head:pull/28228
$ git checkout pull/28228

Update a local copy of the PR:
$ git checkout pull/28228
$ git pull https://git.openjdk.org/jdk.git pull/28228/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 28228

View PR using the GUI difftool:
$ git pr show -t 28228

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/28228.diff

[bridgekeeper]

👋 Welcome back swen! A progress list of the required criteria for merging this PR into pr/26911 will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[openjdk]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[openjdk]

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• core-libs
• hotspot
• security

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