[NFC] cleanup Instruction/Value.findVarDecl() APIs.

These APIs are quite convoluted. The checks for var_decl need to be performed in
just the right order. The is a consequence of complexity in the SIL
representation itself, not a problem with the APIs.

It is common for code to accidentally call a less-complete form of the API. It
is essential that they be defined in a central location, and the we get the same
answer whether we start with an Instruction, Argument, or Value. The primary
public interface should always check for debug_value users. The varDecl property
is actually an implementation detail.

It is questionable whether a function like findVarDecl() that returns a basic
property of SIL and does not require arguments should be a property instead. It
is a function to hint that it may scan the use-list, which is not something
we normally want SIL properties to do. Use-lists can grow linearly in function
size. But, again, this is a natural result of the SIL representation and needs
to be considered an implementation detail.

Author: atrick

SwiftCompilerSources/Sources/Optimizer/FunctionPasses/LifetimeDependenceDiagnostics.swift

@@ -342,7 +342,7 @@ private struct LifetimeVariable {
 
   private init(introducer: Value, _ context: some Context) {
     if let arg = introducer as? FunctionArgument {
-      self.varDecl = arg.varDecl
+      self.varDecl = arg.findVarDecl()
       self.sourceLoc = arg.sourceLoc
       self.isArgument = true
       self.isClosureCapture = arg.isClosureCapture

SwiftCompilerSources/Sources/SIL/Argument.swift

@@ -34,14 +34,14 @@ public class Argument : Value, Hashable {
 
   public var isLexical: Bool { false }
 
-  public var varDecl: VarDecl? {
+  public func findVarDecl() -> VarDecl? {
     if let varDecl = bridged.getVarDecl().getAs(VarDecl.self) {
       return varDecl
     }
-    return debugUserDecl
+    return findVarDeclFromDebugUsers()
   }
 
-  public var sourceLoc: SourceLoc? { varDecl?.nameLoc }
+  public var sourceLoc: SourceLoc? { findVarDecl()?.nameLoc }
 
   public static func ==(lhs: Argument, rhs: Argument) -> Bool {
     lhs === rhs

SwiftCompilerSources/Sources/SIL/Instruction.swift

@@ -450,17 +450,17 @@ public enum VariableScopeInstruction {
     scopeBegin.uses.lazy.filter { $0.endsLifetime || $0.instruction is ExtendLifetimeInst }
   }
 
-  // TODO: with SIL verification, we might be able to make varDecl non-Optional.
-  public var varDecl: VarDecl? {
-    if let debugVarDecl = instruction.debugResultDecl {
-      return debugVarDecl
-    }
+  // TODO: assert that VarDecl is valid whenever isFromVarDecl returns tyue.
+  public func findVarDecl() -> VarDecl? {
     // SILGen may produce double var_decl instructions for the same variable:
     //   %box = alloc_box [var_decl] "x"
     //   begin_borrow %box [var_decl]
     //
-    // Assume that, if the begin_borrow or move_value does not have its own debug_value, then it must actually be
-    // associated with its operand's var_decl.
+    // Therefore, first check if begin_borrow or move_value has any debug_value users.
+    if let debugVarDecl = instruction.findVarDeclFromDebugUsers() {
+      return debugVarDecl
+    }
+    // Otherwise, assume that the var_decl is associated with its operand's var_decl.
     return instruction.operands[0].value.definingInstruction?.findVarDecl()
   }
 }
@@ -472,14 +472,14 @@ extension Instruction {
       return varDeclInst.varDecl
     }
     if let varScopeInst = VariableScopeInstruction(self) {
-      return varScopeInst.varDecl
+      return varScopeInst.findVarDecl()
     }
-    return debugResultDecl
+    return findVarDeclFromDebugUsers()
   }
 
-  var debugResultDecl: VarDecl? {
+  func findVarDeclFromDebugUsers() -> VarDecl? {
     for result in results {
-      if let varDecl = result.debugUserDecl {
+      if let varDecl = result.findVarDeclFromDebugUsers() {
         return varDecl
       }
     }
@@ -488,14 +488,14 @@ extension Instruction {
 }
 
 extension Value {
-  var debugValDecl: VarDecl? {
+  public func findVarDecl() -> VarDecl? {
     if let arg = self as? Argument {
-      return arg.varDecl
+      return arg.findVarDecl()
     }
-    return debugUserDecl
+    return findVarDeclFromDebugUsers()
   }
 
-  var debugUserDecl: VarDecl? {
+  func findVarDeclFromDebugUsers() -> VarDecl? {
     for use in uses {
       if let debugVal = use.instruction as? DebugValueInst {
         return debugVal.varDecl