RFR: 8367656: Refactor Constantpool's operand array into two

[Johan Sjölen]

Hi,

This is a refactoring of the way that we store the Bootstrap method attribute in the ConstantPool class. We used to have a single `Array<u2>` which was divided into a section of `u4` offsets and a section which was the actual data. In this refactoring we make this split more clear, by actually allocating an `Array<u4>` to store the offsets in and an `Array<u2>` to store the data in. These arrays are then put into a `BSMAttributeEntries` class, which allows us to separate out the API from that of the rest of the `ConstantPool`.

We had multiple instances of the code knowing the layout of the operands array and using this to do 'clever' ways of copying and inserting data into it. See `ConstantPool::copy_operands` and `ConstantPool::resize_operands`. I felt like we could do things in a simpler way, so I added the `start_/end_extension` protocol and added the `InsertionIterator` for this. See `ClassFileParser::parse_classfile_bootstrap_methods_attribute` for how this works. I put several relevant definitions into the inline file in hopes of encouraging the compiler to optimize these appropriately.

For the Java SA code, I had to add a `U4Array` class. I also had to fix the vmstructs definitions, etc.

On the whole, while this code is a bit less terse, I think it's a good API improvement and the underlying implementation of splitting up the operands array is also an improvement.

Testing: Oracle Tier1-Tier5 has been run succesfully multiple times. Before integration, I will merge with master and run these tiers again.

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Commit messages:
 - Bug
 - Missing inline
 - oops again
 - Oops
 - Docs
 - Another one
 - Simplify further
 - Rename and simplify
 - This is wron
 - Deleting operands

Changes: https://git.openjdk.org/jdk/pull/27198/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27198&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367656
  Stats: 728 lines in 11 files changed: 321 ins; 247 del; 160 mod
  Patch: https://git.openjdk.org/jdk/pull/27198.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27198/head:pull/27198

PR: https://git.openjdk.org/jdk/pull/27198