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

[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

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

Commit messages:
 - fix 32 bit build
 - 8286972: Support the new loop induction variable related PopulateIndex IR node on x86

Changes: https://git.openjdk.java.net/jdk/pull/8778/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8778&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286972
  Stats: 152 lines in 3 files changed: 130 ins; 21 del; 1 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