RFR: 8367656: Refactor Constantpool's operand array into two [v8]

[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.

Johan Sjölen has updated the pull request incrementally with one additional commit since the last revision:

  Serguei's review

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/27198/files
  - new: https://git.openjdk.org/jdk/pull/27198/files/d8624f09..f96a16e5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27198&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27198&range=06-07

  Stats: 15 lines in 4 files changed: 0 ins; 0 del; 15 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