Fix #4019: Recognize with expressions whose initializer awaits

Roslyn spills the receiver of an initializer assignment that precedes an
await into a hoisted temporary, so the clone of a with expression (and the
object of an object initializer) reaches the initializer transform through
a copy it rejects. Only copies between two hoisted compiler temporaries are
inlined: folding copies of hoisted user locals as well would turn a user's
local into an initializer.

ICSharpCode.Decompiler/IL/Transforms/ILInlining.cs

@@ -668,12 +668,17 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				case OpCode.SwitchInstruction:
 					return true;
 				case OpCode.LdLoc:
-					if (v.StateMachineField == null && ((LdLoc)inlinedExpression).Variable.StateMachineField != null)
+					var copiedVariable = ((LdLoc)inlinedExpression).Variable;
+					if (v.StateMachineField == null && copiedVariable.StateMachineField != null)
 					{
 						// Roslyn likes to put the result of fetching a state machine field into a temporary variable,
 						// so inline more aggressively in such cases.
 						return true;
 					}
+					if (IsHoistedCompilerTemporary(v) && IsHoistedCompilerTemporary(copiedVariable))
+					{
+						return true;
+					}
 					break;
 			}
 			if (inlinedExpression.ResultType == StackType.Ref)
@@ -773,6 +778,13 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			}
 		}
 
+		static bool IsHoistedCompilerTemporary(ILVariable v)
+		{
+			string name = v.StateMachineField?.Name;
+			return name != null
+				&& (name.StartsWith("<>7__wrap", StringComparison.Ordinal) || name.StartsWith("<>s__", StringComparison.Ordinal));
+		}
+
 		/// <summary>
 		/// Gets whether 'expressionBeingMoved' can be inlined into 'expr'.
 		/// </summary>