Require for loop init rewriter for SSA.

Co-authored-by: Harikrishnan Mulackal <webmail.hari@gmail.com>

libyul/optimiser/SSATransform.cpp

@@ -194,7 +194,7 @@ void IntroduceControlFlowSSA::operator()(FunctionDefinition& _function)
 
 void IntroduceControlFlowSSA::operator()(ForLoop& _for)
 {
-	(*this)(_for.pre);
+	yulAssert(_for.pre.statements.empty(), "For loop init rewriter not run.");
 
 	Assignments assignments;
 	assignments(_for.body);
@@ -357,11 +357,7 @@ void PropagateValues::operator()(Assignment& _assignment)
 
 void PropagateValues::operator()(ForLoop& _for)
 {
-	// This will clear the current value in case of a reassignment inside the
-	// init part, although the new variable would still be in scope inside the whole loop.
-	// This small inefficiency is fine if we move the pre part of all for loops out
-	// of the for loop.
-	(*this)(_for.pre);
+	yulAssert(_for.pre.statements.empty(), "For loop init rewriter not run.");
 
 	Assignments assignments;
 	assignments(_for.body);

libyul/optimiser/SSATransform.h

@@ -85,7 +85,7 @@ class NameDispenser;
  *
  * TODO Which transforms are required to keep this idempotent?
  *
- * Prerequisite: Disambiguator.
+ * Prerequisite: Disambiguator, ForLoopInitRewriter.
  */
 class SSATransform: public ASTModifier
 {

test/libyul/YulOptimizerTest.cpp

@@ -255,6 +255,7 @@ TestCase::TestResult YulOptimizerTest::run(ostream& _stream, string const& _line
 	else if (m_optimizerStep == "ssaTransform")
 	{
 		disambiguate();
+		ForLoopInitRewriter::run(*m_context, *m_ast);
 		SSATransform::run(*m_context, *m_ast);
 	}
 	else if (m_optimizerStep == "redundantAssignEliminator")

test/libyul/yulOptimizerTests/ssaTransform/for_def_in_init.yul

@@ -0,0 +1,33 @@
+{
+  for { let x := 0 } 1 { x := 2 } {
+    for { let y := 0 } 1 { y := 6 } {
+    }
+  }
+}
+// ----
+// step: ssaTransform
+//
+// {
+//     let x_1 := 0
+//     let x := x_1
+//     for { }
+//     1
+//     {
+//         let x_7 := x
+//         let x_2 := 2
+//         x := x_2
+//     }
+//     {
+//         let x_5 := x
+//         let y_3 := 0
+//         let y := y_3
+//         for { }
+//         1
+//         {
+//             let y_6 := y
+//             let y_4 := 6
+//             y := y_4
+//         }
+//         { }
+//     }
+// }

test/libyul/yulOptimizerTests/ssaTransform/for_reassign_body.yul

@@ -12,7 +12,8 @@
 // {
 //     let a_1 := mload(0)
 //     let a := a_1
-//     for { mstore(0, a_1) }
+//     mstore(0, a_1)
+//     for { }
 //     a
 //     {
 //         let a_4 := a

test/libyul/yulOptimizerTests/ssaTransform/for_reassign_init.yul

@@ -12,19 +12,9 @@
 // {
 //     let a_1 := mload(0)
 //     let a := a_1
-//     for {
-//         let a_2 := add(a_1, 3)
-//         a := a_2
-//     }
-//     a
-//     {
-//         let a_4 := a
-//         mstore(0, a_4)
-//     }
-//     {
-//         let a_3 := a
-//         mstore(0, a_3)
-//     }
-//     let a_5 := a
-//     mstore(0, a_5)
+//     let a_2 := add(a_1, 3)
+//     a := a_2
+//     for { } a_2 { mstore(0, a_2) }
+//     { mstore(0, a_2) }
+//     mstore(0, a_2)
 // }

test/libyul/yulOptimizerTests/ssaTransform/for_reassign_post.yul

@@ -12,7 +12,8 @@
 // {
 //     let a_1 := mload(0)
 //     let a := a_1
-//     for { mstore(0, a_1) }
+//     mstore(0, a_1)
+//     for { }
 //     a
 //     {
 //         let a_4 := a

test/libyul/yulOptimizerTests/ssaTransform/for_simple.yul

@@ -32,10 +32,9 @@
 //         a := a_4
 //     }
 //     let a_10 := a
-//     for {
-//         let a_5 := add(a_10, 3)
-//         a := a_5
-//     }
+//     let a_5 := add(a_10, 3)
+//     a := a_5
+//     for { }
 //     a
 //     {
 //         let a_12 := a