Add comments to SwiftCompilerSources for shouldExpand()

Author: aschwaighofer

SwiftCompilerSources/Sources/Optimizer/FunctionPasses/DeadStoreElimination.swift

@@ -75,6 +75,7 @@ let deadStoreElimination = FunctionPass(name: "dead-store-elimination") {
 }
 
 private func tryEliminate(store: StoreInst, complexityBudget: inout Int, _ context: FunctionPassContext) {
+  // Check if the type can be expanded without a significant increase to code size.
   if !store.hasValidOwnershipForDeadStoreElimination || !store.source.type.shouldExpand(context) {
     return
   }

SwiftCompilerSources/Sources/Optimizer/FunctionPasses/RedundantLoadElimination.swift

@@ -97,6 +97,8 @@ private func eliminateRedundantLoads(in function: Function, ignoreArrays: Bool,
         {
           continue
         }
+        // Check if the type can be expanded without a significant increase to
+        // code size.
         if !load.type.shouldExpand(context) {
            continue
         }

SwiftCompilerSources/Sources/Optimizer/Utilities/OptUtils.swift

@@ -811,6 +811,11 @@ extension CheckedCastAddrBranchInst {
 }
 
 extension Type {
+  /// True if a type can be expanded without a significant increase to code
+  /// size.
+  ///
+  /// False if expanding a type is invalid. For example, expanding a
+  /// struct-with-deinit drops the deinit.
   func shouldExpand(_ context: some Context) -> Bool {
     if !context.options.useAggressiveReg2MemForCodeSize {
       return true