Fix -Wnonnull warnings with GCC 11

Version 11 and later of GCC consider the implicit `this` parameter of
nonstatic member functions to be annotated with `nonnull` for the
purposes of the `-Wnonnull` warning. This change generates new warnings
that are resolved here with the use of `TR_ASSERT_FATAL` on the objects
that the compiler thinks could be `NULL`. The macro `TR_ASSERT` is
insufficient, as the `TR::assertion` method isn't (and cannot be)
annotated with `OMR_NORETURN`.

Signed-off-by: Christian Despres <despresc@ibm.com>

compiler/infra/InterferenceGraph.cpp

@@ -417,7 +417,7 @@ bool TR_InterferenceGraph::simplify()
  }
  }
 
- TR_ASSERT(bestSpillNode, "Could not find a spill candidate.\n");
+ TR_ASSERT_FATAL(bestSpillNode, "Could not find a spill candidate");
 
  virtualRemoveNodeFromIG(bestSpillNode);
  workingSet->reset(bestSpillNode->getIndex());

compiler/optimizer/CopyPropagation.cpp

@@ -1230,7 +1230,7 @@ void TR_CopyPropagation::replaceCopySymbolReferenceByOriginalIn(TR::SymbolRefere
  if (origNode->getNumChildren() == 2)
  {
  twoChildrenCase = true;
- TR_ASSERT(0, "Only in WCode we can add extra children\n");
+ TR_ASSERT_FATAL(false, "Cannot add extra children");
  }
  else
  {

compiler/optimizer/Structure.cpp

@@ -2737,6 +2737,7 @@ bool TR_RegionStructure::changeContinueLoopsToNestedLoops(TR_RegionStructure *ro
  {
  diagnostic("Incoming [%p] edge %d->%d\n", incomingEdge, incomingEdge->getFrom()->asBlock()->getNumber(), incomingEdge->getTo()->asBlock()->getNumber());
  diagnostic("\tassuming it should be ->%d\n", entryBlock->getNumber());
+ TR_ASSERT_FATAL(newHeader, "New header must have been set by now");
  diagnostic("\tredirecting to ->%d\n", newHeader->getNumber());
  }