RFR: 8340398: [JVMCI] Unintuitive behavior of UseJVMCICompiler option [v2]

[Tomáš Zezula]

> Disabling the JVMCI compiler with `-XX:-UseJVMCICompiler` not only deactivates JVMCI-based CompileBroker compilations but also prevents the loading of the libjvmci compiler. While this works as expected for CompileBroker compilations, it poses issues for the Truffle compiler. When `-XX:-UseJVMCICompiler` is used, Truffle falls back to the jargraal compiler, if available. This behavior may be confusing for Truffle users.
> 
> Expected behavior:
> 
> With `-XX:+UseGraalJIT`, both CompileBroker compilations and Truffle compilations should utilize the libjvmci compiler, if available.
> With `-XX:+EnableJVMCI`, CompileBroker compilations should use the C2 compiler, while only Truffle compilations should leverage the libjvmci compiler, if available.

Tomáš Zezula has updated the pull request incrementally with one additional commit since the last revision:

  JDK-8340398: Fixed EnableJVMCI handling.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/21069/files
  - new: https://git.openjdk.org/jdk/pull/21069/files/78f57619..b7550463

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21069&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21069&range=00-01

  Stats: 15 lines in 1 file changed: 9 ins; 6 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/21069.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21069/head:pull/21069

PR: https://git.openjdk.org/jdk/pull/21069