RFR: 8318457: Use prefix-less prepend methods directly to reduce branches in String concat expressions [v3]

[Claes Redestad]

> In #16006 using `StringConcatFactory` seem to induce a run-to-run variance. One idea to reduce this is to generate expression trees with fewer branches by looking up and using the prefix-less `prepend` methods directly for cases where constant is null/empty so that the prefixed prepender will always deal with a non-null prefix constant.

Claes Redestad has updated the pull request incrementally with one additional commit since the last revision:

  drop dropLookupMode

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16244/files
  - new: https://git.openjdk.org/jdk/pull/16244/files/faa823d3..ca0ac91f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16244&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16244&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/16244.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16244/head:pull/16244

PR: https://git.openjdk.org/jdk/pull/16244