SIL: fix self-argument APIs in Function

`var selfArgument` should return an optional, i.e. nil if the function doesn't have a self argument.

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/Utilities/Devirtualization.swift

@@ -122,7 +122,7 @@ extension DestroyValueInst : DevirtualizableDestroy {
     let builder = Builder(before: self, context)
     let subs = context.getContextSubstitutionMap(for: type)
     let deinitRef = builder.createFunctionRef(deinitializer)
-    if deinitializer.argumentConventions[deinitializer.selfArgumentIndex].isIndirect {
+    if deinitializer.argumentConventions[deinitializer.selfArgumentIndex!].isIndirect {
       let allocStack = builder.createAllocStack(type)
       builder.createStore(source: destroyedValue, destination: allocStack, ownership: .initialize)
       builder.createApply(function: deinitRef, subs, arguments: [allocStack])
@@ -194,7 +194,7 @@ extension DestroyAddrInst : DevirtualizableDestroy {
     let builder = Builder(before: self, context)
     let subs = context.getContextSubstitutionMap(for: destroyedAddress.type)
     let deinitRef = builder.createFunctionRef(deinitializer)
-    if !deinitializer.argumentConventions[deinitializer.selfArgumentIndex].isIndirect {
+    if !deinitializer.argumentConventions[deinitializer.selfArgumentIndex!].isIndirect {
       let value = builder.createLoad(fromAddress: destroyedAddress, ownership: .take)
       builder.createApply(function: deinitRef, subs, arguments: [value])
     } else {

SwiftCompilerSources/Sources/SIL/Effects.swift

@@ -690,10 +690,10 @@ extension StringParser {
   private mutating func parseArgumentIndexFromSource(for function: Function,
                                              params: Dictionary<String, Int>) throws -> Int {
     if consume("self") {
-      if !function.hasSelfArgument {
+      guard let selfArgIdx = function.selfArgumentIndex else {
         try throwError("function does not have a self argument")
       }
-      return function.selfArgumentIndex
+      return selfArgIdx
     }
     if let name = consumeIdentifier() {
       guard let idx = params[name] else {

SwiftCompilerSources/Sources/SIL/Function.swift

@@ -320,10 +320,15 @@ extension Function {
 
   public var hasSelfArgument: Bool { argumentConventions.selfIndex != nil }
 
-  public var selfArgumentIndex: Int { argumentConventions.selfIndex! }
+  public var selfArgumentIndex: Int? { argumentConventions.selfIndex }
+
+  public var selfArgument: FunctionArgument? {
+    if let selfArgIdx = selfArgumentIndex {
+      return arguments[selfArgIdx]
+    }
+    return nil
+  }
 
-  public var selfArgument: FunctionArgument { arguments[selfArgumentIndex] }
-  
   public var dynamicSelfMetadata: FunctionArgument? {
     if bridged.hasDynamicSelfMetadata() {
       return arguments.last!