RFR: 8342826: Improve performance of oopDesc::klass() after JDK-8305895

[Roman Kennke]

Before intergration of JEP 450, a number of minor regressions have been identified. The root-cause of those is the additional flag-check for UseCompactObjectHeaders in a couple of hot code-paths. This change addresses those cases by introducing a new helper class ObjLayout, which initializes some global state depending on the flags, and use that state later, instead of loading and checking multiple flags in hot paths.

This solution is not great. The real fix will eventually be to get rid of UseCompressedClassPointers in a first step, and later also get rid of UseCompactObjectHeaders, and settle on a single object layout. But we are not there, yet, and it will take several (or many) releases to get there. In the meantime, the proposed change eliminates the remaining known regressions.

Relevant benchmarks:

DaCapo:pmd (less is better)
pre-jep450:  703.67
mainline:  729.38
jdk8342826:  704.25

CryptoRsa (more is better)
pre-jep450: 9315.719 ops/min
mainline: 10109.509 ops/min
jdk8342826: 10272.161 ops/min


Throw.throwWith64Frames microbenchmark
Before JEP 450 (605396280d5ea225828da4ed688068334a15e122)
Throw.throwWith64Frames  avgt   40  3943.690 ± 15.456  ns/op
Mainline
Throw.throwWith64Frames  avgt   40  4083.029 ± 12.044  ns/op
JDK-8342826
Throw.throwWith64Frames  avgt   40  3973.082 ± 12.956  ns/op

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

Commit messages:
 - Handle has_klass_gap
 - Rename KlassMode -> ObjLayout
 - Add missing includes
 - 8342826: Improve performance of oopDesc::klass() after JDK-8305895

Changes: https://git.openjdk.org/jdk/pull/22020/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22020&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342826
  Stats: 224 lines in 7 files changed: 189 ins; 8 del; 27 mod
  Patch: https://git.openjdk.org/jdk/pull/22020.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22020/head:pull/22020

PR: https://git.openjdk.org/jdk/pull/22020