#4136: Simplify bool-typed if (c) 1 else 0 to c != 0

An IfInstruction tests its condition for non-zero, so a bool-typed
`if (c) ldc.i4 1 else ldc.i4 0` always equals `c != 0`, which
ExpressionTransforms already folds to `c` for bool `c`. Without this
identity the node reached the C# output as `c ? true : false`, or as
`c || 1 == 0` once negated. The lost `??` form is not addressed here.

ICSharpCode.Decompiler.Tests/TestCases/Pretty/LiftedOperators.cs

@@ -935,6 +935,53 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			//return (a + 1) ?? (b + 2) ?? (c + 3) ?? (d + 4);
 			return 0L;
 		}
+
+#if CS90
+		private static IDisposable Open(int length)
+		{
+			return null;
+		}
+
+		public static void NullOrZeroPatternBeforeUsing(int? num)
+		{
+#if EXPECTED_OUTPUT
+			if (!num.HasValue || num.GetValueOrDefault() == 0)
+#else
+			if (num is null or 0)
+#endif
+			{
+				return;
+			}
+			using (Open(num.Value))
+			{
+				Print(num.Value);
+			}
+		}
+
+		public static void NullOrZeroPatternEarlyReturn(int? num)
+		{
+#if EXPECTED_OUTPUT
+#if OPT && !ROSLYN4
+			if ((num ?? 0) != 0)
+#else
+			if (num.HasValue && num.GetValueOrDefault() != 0)
+#endif
+			{
+				Print(num);
+				Print(num.Value);
+				Print(num.Value + 1);
+			}
+#else
+			if (num is null or 0)
+			{
+				return;
+			}
+			Print(num);
+			Print(num.Value);
+			Print(num.Value + 1);
+#endif
+		}
+#endif
 	}
 
 	internal class T04_LiftedCompoundAssignAndBoxing

ICSharpCode.Decompiler/IL/Transforms/ExpressionTransforms.cs

@@ -584,6 +584,17 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				context.EndStep(matchCondition);
 				return;
 			}
+			if (inst.TrueInst.MatchLdcI4(1) && inst.FalseInst.MatchLdcI4(0)
+				&& inst.InferType(context.TypeSystem).IsKnownType(KnownTypeCode.Boolean))
+			{
+				context.Step("if (c) true else false -> comp(c != 0)", inst);
+				var comp = new Comp(ComparisonKind.Inequality, Sign.None, inst.Condition, new LdcI4(0));
+				comp.AddILRange(inst);
+				inst.ReplaceWith(comp);
+				context.EndStep(comp);
+				comp.AcceptVisitor(this);
+				return;
+			}
 		}
 
 		IfInstruction HandleConditionalOperator(IfInstruction inst)