RFR: 8265911: assert(comp != __null) failed: Compiler instance missing

[Frank King]

As I debugged into the code, the `_c1_count` was set to `1` [1] with flags `-XX:+EnableJVMCI -XX:+UseJVMCICompiler -XX:JVMCIHostThreads=1`. However, [2] would never be executed, and thus `_compilers[0]` was `NULL`. 

When C1 is disabled, `_c1_count` should always be `0`, so I added this `#ifdef` guard to avoid changing value of `_c1_count` under this circumstance.

[1] https://github.com/openjdk/jdk/blob/f1f2afda5a0ab8f213e8a1b5324a251928c8d81a/src/hotspot/share/compiler/compileBroker.cpp#L656
[2] https://github.com/openjdk/jdk/blob/f1f2afda5a0ab8f213e8a1b5324a251928c8d81a/src/hotspot/share/compiler/compileBroker.cpp#L664

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Commit messages:
 - 8265911: assert(comp != __null) failed: Compiler instance missing

Changes: https://git.openjdk.java.net/jdk/pull/3672/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3672&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265911
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3672.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3672/head:pull/3672

PR: https://git.openjdk.java.net/jdk/pull/3672