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

[Tomáš Zezula]

Disabling the JVMCI compiler with `-XX:-UseJVMCICompiler` not only deactivates JVMCI-based host method compilations but also prevents the loading of the libjvmci compiler. While this works as expected for host method 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 host method compilations and Truffle compilations should utilize the libjvmci compiler, if available.
With `-XX:+EnableJVMCI`, host method compilations should use the C2 compiler, while only Truffle compilations should leverage the libjvmci compiler, if available.

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Commit messages:
 - JDK-8340398: [JVMCI] Unintuitive behavior of UseJVMCICompiler option

Changes: https://git.openjdk.org/jdk/pull/21069/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21069&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340398
  Stats: 9 lines in 2 files changed: 6 ins; 0 del; 3 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