RFR: 8263532: C2: Improve vector box elimination for Phi nodes

[Vladimir Ivanov]

Enhance vector box elimination when merging VectorBox nodes through Phis.

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Commit messages:
 - Merge branch 'master' into vector.phi
 - Enhance vector box elimination when merging VectorBox nodes through Phis

Changes: https://git.openjdk.java.net/panama-vector/pull/50/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-vector&pr=50&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263532
  Stats: 94 lines in 2 files changed: 60 ins; 1 del; 33 mod
  Patch: https://git.openjdk.java.net/panama-vector/pull/50.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-vector pull/50/head:pull/50

PR: https://git.openjdk.java.net/panama-vector/pull/50