RFR [sh/8u]: Use correct flag to guard implicit concurrent GC

[Roman Kennke]

This is an 8-only bug. We're using ExplicicGCInvokesConcurrent to guard 
implicit-to-conc GC, it should be ShenandoahImplicicGCInvokesConcurrent 
instead like in 11+

Testing: hotspot_gc_shenandoah

Ok?

diff -r 24f427395b33 -r 57ff1d0cfffe 
src/share/vm/gc_implementation/shenandoah/shenandoahControlThread.cpp
--- 
a/src/share/vm/gc_implementation/shenandoah/shenandoahControlThread.cpp 
Mon Sep 30 18:02:24 2019 +0200
+++ 
b/src/share/vm/gc_implementation/shenandoah/shenandoahControlThread.cpp 
Tue Oct 29 13:38:25 2019 +0100
@@ -170,7 +170,7 @@

        heuristics->record_requested_gc();

-      if (ExplicitGCInvokesConcurrent) {
+      if (ShenandoahImplicitGCInvokesConcurrent) {
          policy->record_implicit_to_concurrent();
          mode = concurrent_normal;