Add flag to disable loadaddr specialization in x86 MemoryReferences

Presently in the x86 code generator, there is an optimization during
MemoryReference creation that always assumes that accesses to local
objects via a loadaddr should use the frame register, as opposed
to evaluating (or re-using) the register associated with the loadaddr
as the base register.

Add an environment variable `TR_useLoadAddrRegisterForLocalObjectMemRef`
to disable this optimization for diagnostic purposes.

Signed-off-by: Daryl Maier <maier@ca.ibm.com>

compiler/x/codegen/OMRMemoryReference.cpp

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 2000, 2020 IBM Corp. and others
+ * Copyright (c) 2000, 2021 IBM Corp. and others
  *
  * This program and the accompanying materials are made available under
  * the terms of the Eclipse Public License 2.0 which accompanies this
@@ -189,7 +189,15 @@ OMR::X86::MemoryReference::MemoryReference(
          // We can't do this when the access is unresolved.
          //
          TR::Node *base = rootLoadOrStore->getFirstChild();
+
+         // Use the register evaluated from the loadaddr as the base object
+         // in a memory reference rather than accessing directly from the
+         // stack.
+         //
+         static bool useLoadAddrRegisterForLocalObjectMemRef = feGetEnv("TR_useLoadAddrRegisterForLocalObjectMemRef") ? true : false;
+
          if (!isUnresolved &&
+             !useLoadAddrRegisterForLocalObjectMemRef &&
              base->getOpCodeValue() == TR::loadaddr &&
              base->getSymbol()->isLocalObject())
             {