RFR: 8329433: Reduce nmethod header size [v3]

[Vladimir Kozlov]

> This is part of changes which try to reduce size of `nmethod` and `codeblob` data vs code in CodeCache.
> These changes reduced size of `nmethod` header from 288 to 240 bytes. From 304 to 256 in optimized VM:
> 
> Statistics for 1282 bytecoded nmethods for C2:
>  total in heap = 5560352 (100%)
>  header = 389728 (7.009053%)
> 
> vs
> 
> Statistics for 1298 bytecoded nmethods for C2:
>  total in heap  = 5766040 (100%)
>  header         = 332288 (5.762846%)
> 
> 
> Several unneeded fields in `nmethod` and `CodeBlob` were removed. Some fields were changed from `int` to `int16_t` with added corresponding asserts to make sure their values are fit into 16 bits.
> 
> I did additional cleanup after recent `CompiledMethod` removal.
> 
> Tested tier1-7,stress,xcomp and performance testing.

Vladimir Kozlov has updated the pull request incrementally with one additional commit since the last revision:

  Union fields which usages do not overlap

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18768/files
  - new: https://git.openjdk.org/jdk/pull/18768/files/488f1b92..13744e78

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18768&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18768&range=01-02

  Stats: 31 lines in 2 files changed: 15 ins; 13 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/18768.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18768/head:pull/18768

PR: https://git.openjdk.org/jdk/pull/18768