RFR: 8282874: Bad performance on gather/scatter API caused by different IntSpecies of indexMap

[Joshua Zhu]

I came across a performance issue when using scatter store VectorAPI for Integer and Long in the same application. The poor performance was caused by vector intrinsic inlining failure because of non-determined IntSpecies for a constant VectorShape of IndexMap in this scenario.
As discussion at https://github.com/openjdk/jdk/pull/7721 , I change the code in VectorAPI.
Please help review.

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Commit messages:
 - 8282874: Bad performance on gather/scatter API caused by different IntSpecies of indexMap

Changes: https://git.openjdk.java.net/jdk/pull/7757/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7757&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282874
  Stats: 42 lines in 7 files changed: 0 ins; 0 del; 42 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7757.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7757/head:pull/7757

PR: https://git.openjdk.java.net/jdk/pull/7757