RFR: JDK-8218159: ValueBootstrapMethods.isSubstitutable should not filter synthetic fields

[Mandy Chung]

`acmp` on two primitive objects of the same type are considered equals if all of their fields are "equivalent" including synthetic fields, for example, which can be an enclosing instance and captured outer locals.

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

Commit messages:
 - JDK-8218159: ValueBootstrapMethods.isSubstitutable should not filter synthetic fields

Changes: https://git.openjdk.java.net/valhalla/pull/454/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=454&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8218159
  Stats: 84 lines in 3 files changed: 80 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/454.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/454/head:pull/454

PR: https://git.openjdk.java.net/valhalla/pull/454