[vectorIntrinsics] RFR: 8252943: [vector] jdk/incubator/vector/PreferredSpeciesTest.java fails with "expected [S_256_BIT] but found [S_128_BIT]"

[Sandhya Viswanathan]

This is a test bug. Updated test for ofLargestShape in PreferredSpeciesTest.java to match the specification.

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Commit messages:
 - Improve test for ofLargestShape
 - 8252943: [vector] jdk/incubator/vector/PreferredSpeciesTest.java fails with "expected [S_256_BIT] but found [S_128_BIT]"

Changes: https://git.openjdk.java.net/panama-vector/pull/16/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-vector&pr=16&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252943
  Stats: 12 lines in 1 file changed: 9 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/panama-vector/pull/16.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-vector pull/16/head:pull/16

PR: https://git.openjdk.java.net/panama-vector/pull/16