DeadStoreElimination: don't assume that the operand of an dealloc_stack is an alloc_stack.

It can also be a `partial_apply`.
Fixes a compiler crash
swiftlang#81698
rdar://151822502

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/FunctionPasses/DeadStoreElimination.swift

@@ -256,7 +256,7 @@ private extension Deallocation {
 
 private extension DeallocStackInst {
   func isStackDeallocation(of base: AccessBase) -> Bool {
-    if case .stack(let allocStack) = base, allocstack == allocStack {
+    if case .stack(let allocStack) = base, operand.value == allocStack {
       return true
     }
     return false

SwiftCompilerSources/Sources/SIL/Instruction.swift

@@ -647,11 +647,7 @@ extension Deallocation {
 }
 
 
-final public class DeallocStackInst : Instruction, UnaryInstruction, Deallocation {
-  public var allocstack: AllocStackInst {
-    return operand.value as! AllocStackInst
-  }
-}
+final public class DeallocStackInst : Instruction, UnaryInstruction, Deallocation {}
 
 final public class DeallocStackRefInst : Instruction, UnaryInstruction, Deallocation {
   public var allocRef: AllocRefInstBase { operand.value as! AllocRefInstBase }