RFR: 8335182: Consolidate and streamline String concat code shapes

[Claes Redestad]

This PR attains a speed-up on some microbenchmarks by simplifying how we build up the MH combinator tree shape
(only use prependers with prefix, always add a suffix to the newArray combinator), then simplifying/inlining some of the code in the helper functions. 

Many simple concatenation expressions stay performance neutral, while the win comes from enabling C2 to better optimize more complex shapes (concat13String, concatMix4String, concatConst6String see relatively large improvements):


Name                                    Cnt     Base     Error      Test     Error  Unit  Change
StringConcat.concat13String              50   66.380 ±   0.189    53.017 ±   0.241 ns/op   1.25x (p = 0.000*)
StringConcat.concat4String               50   13.620 ±   0.053    12.382 ±   0.089 ns/op   1.10x (p = 0.000*)
StringConcat.concat6String               50   17.168 ±   0.070    16.046 ±   0.064 ns/op   1.07x (p = 0.000*)
StringConcat.concatConst2String          50    9.820 ±   0.081     8.158 ±   0.041 ns/op   1.20x (p = 0.000*)
StringConcat.concatConst4String          50   14.938 ±   0.124    12.800 ±   0.049 ns/op   1.17x (p = 0.000*)
StringConcat.concatConst6Object          50   56.115 ±   0.288    54.046 ±   0.214 ns/op   1.04x (p = 0.000*)
StringConcat.concatConst6String          50   19.032 ±   0.073    16.213 ±   0.093 ns/op   1.17x (p = 0.000*)
StringConcat.concatConstBoolByte         50    8.486 ±   0.066     8.556 ±   0.050 ns/op   0.99x (p = 0.004*)
StringConcat.concatConstInt              50    8.942 ±   0.052     7.677 ±   0.029 ns/op   1.16x (p = 0.000*)
StringConcat.concatConstIntConstInt      50   12.883 ±   0.070    12.431 ±   0.070 ns/op   1.04x (p = 0.000*)
StringConcat.concatConstString           50    7.523 ±   0.050     7.486 ±   0.044 ns/op   1.00x (p = 0.058 )
StringConcat.concatConstStringConstInt   50   11.961 ±   0.032    11.699 ±   0.049 ns/op   1.02x (p = 0.000*)
StringConcat.concatEmptyConstInt         50    7.778 ±   0.038     7.723 ±   0.037 ns/op   1.01x (p = 0.000*)
StringConcat.concatEmptyConstString      50    3.506 ±   0.026     3.505 ±   0.015 ns/op   1.00x (p = 0.930 )
StringConcat.concatEmptyLeft             50    3.573 ±   0.075     3.518 ±   0.057 ns/op   1.02x (p = 0.044 )
StringConcat.concatEmptyRight            50    3.713 ±   0.049     3.622 ±   0.053 ns/op   1.02x (p = 0.000*)
StringConcat.concatMethodConstString     50    7.418 ±   0.030     7.478 ±   0.066 ns/op   0.99x (p = 0.005*)
StringConcat.concatMix4String            50   89.243 ±   0.436    71.866 ±   0.894 ns/op   1.24x (p = 0.000*)
StringConcatStartup.MixedLarge.run       10  655.436 ±  29.787   649.730 ±  26.053 ms/op   1.01x (p = 0.500 )
StringConcatStartup.MixedSmall.run       20   51.676 ±   2.324    50.724 ±   5.050 ms/op   1.02x (p = 0.512 )
StringConcatStartup.StringLarge.run      10  166.022 ±  15.672   165.300 ±  14.433 ms/op   1.00x (p = 0.873 )
StringConcatStartup.StringSingle.run     40    0.168 ±   0.016     0.178 ±   0.024 ms/op   0.94x (p = 0.234 )
* = significant


Startup-wise it's more or less neutral as evidenced by the added `StringConcatStartup` micro (a simplified and JMH:ified version of some startup stress tests I've been tinkering with over the years). This PR does not change the total number of classes generated and loaded by this test (3359 total, 2499 generated).

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

Commit messages:
 - Add StringConcatStartup JMH
 - Merge branch 'master' into consolidated_prependers
 - Simplify
 - Remove getBytes changes
 - Streamline newArray
 - Consolidate to always end with newArrayWithSuffix
 - Merge branch 'master' into consolidated_prependers
 - Streamline prependers
 - Fix compilation errors from removing non-prefixed prepend methods
 - Remove non-prefix prepender methods, inline logic into prefixed variant
 - ... and 1 more: https://git.openjdk.org/jdk/compare/efb905e5...6525e945

Changes: https://git.openjdk.org/jdk/pull/19927/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19927&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335182
  Stats: 565 lines in 6 files changed: 412 ins; 114 del; 39 mod
  Patch: https://git.openjdk.org/jdk/pull/19927.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19927/head:pull/19927

PR: https://git.openjdk.org/jdk/pull/19927