RFR: 8287517: C2: assert(vlen_in_bytes == 64) failed: 2

[Sandhya Viswanathan]

Fixed the assertion in load_iota_indices when the length passed is less than 4.
Also fixed the missing break in x86.ad match_rule_supported_vector() for PopulateIndex case.

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Commit messages:
 - 8287517: C2: assert(vlen_in_bytes == 64) failed: 2

Changes: https://git.openjdk.java.net/jdk/pull/8961/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8961&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287517
  Stats: 2 lines in 2 files changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8961.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8961/head:pull/8961

PR: https://git.openjdk.java.net/jdk/pull/8961