RFR: 8318457: Use prefix-less prepend methods directly to reduce branches in String concat expressions [v2]

[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 two additional commits since the last revision:

 - dropLookupMode PRIVATE to ensure we don't accidentally use private methods
 - Make all prepend methods package-private

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/16244/files
  - new: https://git.openjdk.org/jdk/pull/16244/files/4e7dfc0f..faa823d3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16244&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16244&range=00-01

  Stats: 10 lines in 1 file changed: 2 ins; 1 del; 7 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