ValueDefUseWalker: fix a problem with cond_br

Don't crash when visiting the cond_br condition operand.

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/Utilities/WalkUtils.swift

@@ -359,11 +359,14 @@ extension ValueDefUseWalker {
         return .continueWalk
       }
     case let cbr as CondBranchInst:
-      let val = cbr.getArgument(for: operand)
-      if let path = walkDownCache.needWalk(for: val, path: path) {
-        return walkDownUses(ofValue: val, path: path)
+      if let val = cbr.getArgument(for: operand) {
+        if let path = walkDownCache.needWalk(for: val, path: path) {
+          return walkDownUses(ofValue: val, path: path)
+        } else {
+          return .continueWalk
+        }
       } else {
-        return .continueWalk
+        return leafUse(value: operand, path: path)
       }
     case let se as SwitchEnumInst:
       if let (caseIdx, path) = path.pop(kind: .enumCase),

SwiftCompilerSources/Sources/SIL/Instruction.swift

@@ -759,6 +759,7 @@ final public class TryApplyInst : TermInst, FullApplySite {
 final public class BranchInst : TermInst {
   public var targetBlock: BasicBlock { BranchInst_getTargetBlock(bridged).block }
 
+  /// Returns the target block argument for the cond_br `operand`.
   public func getArgument(for operand: Operand) -> Argument {
     return targetBlock.arguments[operand.index]
   }
@@ -776,8 +777,15 @@ final public class CondBranchInst : TermInst {
     return ops[(CondBranchInst_getNumTrueArgs(bridged) &+ 1)..<ops.count]
   }
 
-  public func getArgument(for operand: Operand) -> Argument {
-    let argIdx = operand.index - 1
+  /// Returns the true or false block argument for the cond_br `operand`.
+  ///
+  /// Return nil if `operand` is the condition itself.
+  public func getArgument(for operand: Operand) -> Argument? {
+    let opIdx = operand.index
+    if opIdx == 0 {
+      return nil
+    }
+    let argIdx = opIdx - 1
     let numTrueArgs = CondBranchInst_getNumTrueArgs(bridged)
     if (0..<numTrueArgs).contains(argIdx) {
       return trueBlock.arguments[argIdx]