RFR: 8325169: Reduce String::indexOf overheads [v3]

[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 two additional commits since the last revision:

 - Update comments regarding bounds checks
 - Clamp fromIndex to be in range

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/17685/files
  - new: https://git.openjdk.org/jdk/pull/17685/files/aad343b3..097fc981

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17685&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17685&range=01-02

  Stats: 5 lines in 3 files changed: 0 ins; 0 del; 5 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