RFR: 8325805: Compiler Implementation for Flexible Constructor Bodies (Second Preview) [v8]

[Archie Cobbs]

> This patch changes the compiler to allow constructors to make assignments to fields prior to the `super()` invocation, i.e., in the pre-construction context.  This is part of the work associated with [JDK-8325803 - Flexible Constructor Bodies (Second Preview)](https://bugs.openjdk.org/browse/JDK-8325803).
> 
> This patch is based the relevant bits from @vicente-romero-oracle's commit [1b99b5cf](https://github.com/openjdk/valhalla/commit/1b99b5cfd9a5d484b9e7bdfc284daad9ad6535bf) to the valhalla repo.

Archie Cobbs has updated the pull request incrementally with one additional commit since the last revision:

  Update feature name "statements before super()" -> "flexible constructors".

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Changes:
  - all: https://git.openjdk.org/jdk/pull/18088/files
  - new: https://git.openjdk.org/jdk/pull/18088/files/619ab2b2..51b16d3d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18088&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18088&range=06-07

  Stats: 72 lines in 6 files changed: 33 ins; 33 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/18088.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18088/head:pull/18088

PR: https://git.openjdk.org/jdk/pull/18088