[flang] Rewrite "1*j" to "(j)", not "j", when j is a variable

[klausler]

Summary

Expression folding currently unconditionally rewrites "1*j"
to "j", which is wrong when "j" is a variable, as it transforms
an expression into a variable and can lead to incorrect associations
in contexts like an actual argument or an ASSOCIATE selector.
Transform "1*j" to a parenthesized "(j)" when "j" is a variable.
Fixes LLVM bug https://github.com/llvm/llvm-project/issues/63259.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Evaluate/fold-implementation.h

 
 template <typename T>
 Expr<T> FoldOperation(FoldingContext &context, Negate<T> &&x) {
   if (auto array{ApplyElementwise(context, x)}) {
     return *array;
   }
   auto &operand{x.left()};
   if (auto *nn{std::get_if<Negate<T>>(&x.left().u)}) {
-    return std::move(nn->left()); // -(-x) -> x
+    // -(-x) -> (x)
+    if (IsVariable(nn->left())) {
+      return FoldOperation(context, Parentheses<T>{std::move(nn->left())});
+    } else {
+      return std::move(nn->left());
+    }
   } else if (auto value{GetScalarConstantValue<T>(operand)}) {
     if constexpr (T::category == TypeCategory::Integer) {
       auto negated{value->Negate()};
       if (negated.overflow) {
         context.messages().Say(
             "INTEGER(%d) negation overflowed"_warn_en_US, T::kind);
       }
       return Expr<T>{Constant<T>{std::move(negated.value)}};
if (auto folded{OperandsAreConstants(x)}) {
     if (auto c{GetScalarConstantValue<T>(x.right())}) {
       x.right() = std::move(x.left());
       x.left() = Expr<T>{std::move(*c)};
     }
     if (auto c{GetScalarConstantValue<T>(x.left())}) {
       if (c->IsZero()) {
         return std::move(x.left());
       } else if (c->CompareSigned(Scalar<T>{1}) == Ordering::Equal) {
+        if (IsVariable(x.right())) {
+          return FoldOperation(context, Parentheses<T>{std::move(x.right())});
+        } else {
           return std::move(x.right());
+        }
       } else if (c->CompareSigned(Scalar<T>{-1}) == Ordering::Equal) {
-        return Expr<T>{Negate<T>{std::move(x.right())}};
+        return FoldOperation(context, Negate<T>{std::move(x.right())});
       }
     }
   }
   return Expr<T>{std::move(x)};
 }
 
 template <typename T>
 Expr<T> FoldOperation(FoldingContext &context, Divide<T> &&x) {

flang/test/Evaluate/rewrite04.f90

+! RUN: %flang_fc1 -fdebug-unparse %s 2>&1 | FileCheck %s
+! Ensure folding of 1*j is a parenthesized (j) when j is a variable.
+call foo(1*j)
+!CHECK: CALL foo((j))
+end