RFR: 8341548: More concise use of classfile API [v2]

[Shaojin Wen]

> java.base should provide best practices for Class File API
> 
> 1. Use fluent coding style
> 2. Use aconst_null instead of oadConstant(null)
> 3. use astore intead of 'storeLocal(REFERENCE'
> 4. use aload instead of 'loadLocal(REFERENCE'
> 5. 'lload/lstore' instead of 'loadLocal(LONG)/storeLocal(LONG)'

Shaojin Wen has updated the pull request incrementally with one additional commit since the last revision:

  more fluent coding style

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/21355/files
  - new: https://git.openjdk.org/jdk/pull/21355/files/8b2fa4d0..4d895f45

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21355&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21355&range=00-01

  Stats: 21 lines in 2 files changed: 5 ins; 4 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/21355.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21355/head:pull/21355

PR: https://git.openjdk.org/jdk/pull/21355