RFR: 8320448: Accelerate IndexOf using AVX2 [v52]

[Scott Gibbons]

> Re-write the IndexOf code without the use of the pcmpestri instruction, only using AVX2 instructions.  This change accelerates String.IndexOf on average 1.3x for AVX2.  The benchmark numbers:
> 
> 
> Benchmark	                                               Score		Latest		
> StringIndexOf.advancedWithMediumSub   343.573		317.934		0.925375393x
> StringIndexOf.advancedWithShortSub1	  1039.081		1053.96		1.014319384x
> StringIndexOf.advancedWithShortSub2	      55.828		110.541		1.980027943x
> StringIndexOf.constantPattern	                9.361		11.906		1.271872663x
> StringIndexOf.searchCharLongSuccess	        4.216		4.218		1.000474383x
> StringIndexOf.searchCharMediumSuccess	3.133		3.216		1.02649218x
> StringIndexOf.searchCharShortSuccess	3.76		        3.761		1.000265957x
> StringIndexOf.success	                                9.186		9.713		1.057369911x
> StringIndexOf.successBig	                      14.341		46.343		3.231504079x
> StringIndexOfChar.latin1_AVX2_String	  6220.918		12154.52		1.953814533x
> StringIndexOfChar.latin1_AVX2_char	  5503.556		5540.044		1.006629895x
> StringIndexOfChar.latin1_SSE4_String	  6978.854		6818.689		0.977049957x
> StringIndexOfChar.latin1_SSE4_char	  5657.499		5474.624		0.967675646x
> StringIndexOfChar.latin1_Short_String	  7132.541		6863.359		0.962260014x
> StringIndexOfChar.latin1_Short_char	16013.389	      16162.437		1.009307711x
> StringIndexOfChar.latin1_mixed_String	  7386.123	      14771.622		1.999915517x
> StringIndexOfChar.latin1_mixed_char	  9901.671		9782.245		0.987938803

Scott Gibbons has updated the pull request incrementally with one additional commit since the last revision:

  Fix copyright & a couple of comment typos

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/16753/files
  - new: https://git.openjdk.org/jdk/pull/16753/files/6eae46e5..f432320f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16753&range=51
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16753&range=50-51

  Stats: 5 lines in 2 files changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/16753.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16753/head:pull/16753

PR: https://git.openjdk.org/jdk/pull/16753