RFR: 8286972: Support the new loop induction variable related PopulateIndex IR node on x86 [v3]

[Sandhya Viswanathan]

> This PR adds x86 backend support for the new loop induction variable related PopulateIndex IR node. 
> This IR node was added as part of [JDK-8280510](https://bugs.openjdk.java.net/browse/JDK-8280510).
> 
> The performance numbers are as follows:
> Before:
> Benchmark                   (count)   Mode  Cnt       Score       Error  Units
> IndexVector.exprWithIndex1    65536  thrpt    3   64556.552 ±  1126.396  ops/s
> IndexVector.exprWithIndex2    65536  thrpt    3   22117.050 ± 11452.098  ops/s
> IndexVector.indexArrayFill    65536  thrpt    3  117776.383 ±  1120.957  ops/s
> 
> After:
> Benchmark                   (count)   Mode  Cnt       Score       Error  Units
> IndexVector.exprWithIndex1    65536  thrpt    3  203180.290 ±  2147.807  ops/s
> IndexVector.exprWithIndex2    65536  thrpt    3  274132.756 ±  6853.393  ops/s
> IndexVector.indexArrayFill    65536  thrpt    3  374165.202 ± 46930.779  ops/s
> 
> Please review.
> 
> Best Regards,
> Sandhya

Sandhya Viswanathan has updated the pull request incrementally with two additional commits since the last revision:

 - remove warmup
 - Add jtreg test

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8778/files
  - new: https://git.openjdk.java.net/jdk/pull/8778/files/ab07fae9..727daec0

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8778&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8778&range=01-02

  Stats: 114 lines in 1 file changed: 114 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8778.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8778/head:pull/8778

PR: https://git.openjdk.java.net/jdk/pull/8778