Fixup private linkage preprologue calculation

The `preProcInstruction` is the instruction that was before the JIT to JIT entry point before the arguments are loaded from the stack. This is the instruction we want to use as the entry point into the method.

Signed-off-by: Filip Jeremic <fjeremic@ca.ibm.com>

compiler/z/codegen/OMRCodeGenerator.cpp

@@ -2493,7 +2493,7 @@ OMR::Z::CodeGenerator::doBinaryEncoding()
 
       self()->addToAtlas(data.cursorInstruction);
 
-      if (data.cursorInstruction->getOpCodeValue() == TR::InstOpCode::PROC)
+      if (data.cursorInstruction == data.preProcInstruction)
          {
          self()->setPrePrologueSize(self()->getBinaryBufferLength());
          self()->comp()->getSymRefTab()->findOrCreateStartPCSymbolRef()->getSymbol()->getStaticSymbol()->setStaticAddress(self()->getBinaryBufferCursor());