RFR (S) 8225342: Shenandoah: use COMPILER2_OR_JVMCI macro consistently

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8225342

Webrev:
  http://cr.openjdk.java.net/~shade/8225342/webrev.01/

This is cleaner and safer (in case COMPILER2 is actually globally defined but not available in
current compilation unit).

Testing: hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey

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