Fix #3468: Try harder to avoid ref locals if UseRefLocalsForAccurateOrderOfEvaluation is not enabled.

CopyPropagation will replace `ref StructWithStringField reference = ref array[0];` with:
```
var x = array;
var y = 0;
```
and then every use of `reference` is replaced with `x[y]`.

This lets us avoid rough locals while preserving the semantics in every case except that we re-order when a NullReferenceException/IndexOutOfRangeException occurs.

Author: dgrunwald

ICSharpCode.Decompiler/IL/Transforms/CopyPropagation.cs

@@ -87,15 +87,15 @@ static void RunOnBlock(Block block, ILTransformContext context, HashSet<ILVariab
 							block.Instructions[i] = copiedExpr;
 						}
 					}
-					else if (v.IsSingleDefinition && CanPerformCopyPropagation(v, copiedExpr, splitVariables))
+					else if (v.IsSingleDefinition && CanPerformCopyPropagation(v, copiedExpr, splitVariables, context.Settings))
 					{
 						DoPropagate(v, copiedExpr, block, ref i, context);
 					}
 				}
 			}
 		}
 
-		static bool CanPerformCopyPropagation(ILVariable target, ILInstruction value, HashSet<ILVariable> splitVariables)
+		static bool CanPerformCopyPropagation(ILVariable target, ILInstruction value, HashSet<ILVariable> splitVariables, DecompilerSettings settings)
 		{
 			Debug.Assert(target.StackType == value.ResultType);
 			if (target.Type.IsSmallIntegerType())
@@ -107,14 +107,15 @@ static bool CanPerformCopyPropagation(ILVariable target, ILInstruction value, Ha
 			switch (value.OpCode)
 			{
 				case OpCode.LdLoca:
-				//				case OpCode.LdElema:
-				//				case OpCode.LdFlda:
 				case OpCode.LdsFlda:
 					// All address-loading instructions always return the same value for a given operand/argument combination,
 					// so they can be safely copied.
 					// ... except for LdElema and LdFlda, because those might throw an exception, and we don't want to
 					// change the place where the exception is thrown.
 					return true;
+				case OpCode.LdElema:
+				case OpCode.LdFlda:
+					return !settings.UseRefLocalsForAccurateOrderOfEvaluation;
 				case OpCode.LdLoc:
 					var v = ((LdLoc)value).Variable;
 					if (splitVariables != null && splitVariables.Contains(v))