RFR: 8325169: Reduce String::indexOf overheads [v2]

[Claes Redestad]

> This patch streamlines and specializes various `String::indexOf` methods. Mainly avoids the need for clamping and doing checks that are redundant in almost all cases, moving the checks to the API boundary where they are needed. 
> 
> This improves performance both at peak and during startup/warmup. Since indexOf is heavily used in bootstrapping code it makes sense to slightly dial back abstraction and delegation, which in this case also brought some benefit to peak performance.
> 
> Testing: tier1-3

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

  Add comment about preconditions

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17685/files
  - new: https://git.openjdk.org/jdk/pull/17685/files/067f1ea6..aad343b3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17685&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17685&range=00-01

  Stats: 2 lines in 2 files changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17685.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17685/head:pull/17685

PR: https://git.openjdk.org/jdk/pull/17685