RFR: 8270519: Move several vector helper methods to shared header file

[Xiaohong Gong]

Hi, could anyone please take a look at this simple patch?

This is a followed-up patch to [1] which added several arch independent vector methods (e.g. `vector_length/vector_element_basic_type`). Since these methods also exist in x86.ad and are widely used in their match rules, it's better to move the shared methods to a common shared place.

[1] https://bugs.openjdk.java.net/browse/JDK-8269725

Tested tier1 and jdk:tier3.

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Commit messages:
 - 8270519: Move several vector helper methods to shared header file

Changes: https://git.openjdk.java.net/jdk/pull/4822/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4822&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270519
  Stats: 480 lines in 6 files changed: 45 ins; 57 del; 378 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4822.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4822/head:pull/4822

PR: https://git.openjdk.java.net/jdk/pull/4822