Add support to while loops in OpenQASM3 frontend (#5)

This PR adds support to while loop in OpenQASM3 frontend. 

For a OpenQASM3 program:
```qasm
OPENQASM 3.0;

gate h q {
    U(1.57079632679, 0.0, 3.14159265359) q;
}

qubit $0;
int n = 5;

while (n > 0) {
    h $0;
    // error: Binary operation ASTOpTypeSub not supported yet.
    // n = n - 1;
}
```

generates MLIR:
```mlir
...
scf.while : () -> () {
    %2 = quir.use_variable @n : i32
    %c0_i32_0 = arith.constant 0 : i32
    %3 = arith.cmpi sgt, %2, %c0_i32_0 : i32
    scf.condition(%3)
} do {
    quir.call_gate @h(%0) : (!quir.qubit<1>) -> ()
    %cst = constant unit
    scf.yield
    }
}
...
```

lib/Frontend/OpenQASM3/QUIRGenQASM3Visitor.cpp

@@ -361,7 +361,7 @@ void QUIRGenQASM3Visitor::visit(const ASTIfStatementNode *node) {
 }
 
 void QUIRGenQASM3Visitor::visit(const ASTElseStatementNode *node) {
-  // This is processed with the IfStatementNode.
+  // This is processed with the ASTIfStatementNode.
 }
 
 void QUIRGenQASM3Visitor::visit(const ASTSwitchStatementNode *node) {
@@ -443,13 +443,29 @@ void QUIRGenQASM3Visitor::visit(const ASTSwitchStatementNode *node) {
 }
 
 void QUIRGenQASM3Visitor::visit(const ASTWhileStatementNode *node) {
-  reportError(node, mlir::DiagnosticSeverity::Error)
-      << "While loops are not yet supported.";
+  const ASTWhileLoopNode *loop = node->GetLoop();
+  Location loc = getLocation(node);
+
+  auto whileOp =
+      builder.create<scf::WhileOp>(loc, TypeRange({}), ValueRange({}));
+  builder.createBlock(&whileOp.getBefore());
+
+  const ASTExpressionNode *exprNode = loop->GetExpression();
+  Value condition = visitAndGetExpressionValue(exprNode);
+
+  builder.create<scf::ConditionOp>(loc, condition, ValueRange({}));
+
+  builder.createBlock(&whileOp.getAfter());
+
+  const ASTStatementList &statementList = loop->GetStatementList();
+  BaseQASM3Visitor::visit(&statementList);
+
+  builder.create<scf::YieldOp>(loc);
+  builder.setInsertionPointAfter(whileOp);
 }
 
 void QUIRGenQASM3Visitor::visit(const ASTWhileLoopNode *node) {
-  reportError(node, mlir::DiagnosticSeverity::Error)
-      << "While loops are not yet supported.";
+  // This is processed with the ASTWhileStatementNode.
 }
 
 void QUIRGenQASM3Visitor::visit(const ASTReturnStatementNode *node) {

test/Frontend/OpenQASM3/while-1.qasm

@@ -0,0 +1,46 @@
+OPENQASM 3.0;
+// RUN: qss-compiler -X=qasm --emit=ast-pretty %s | FileCheck %s --match-full-lines --check-prefix AST-PRETTY
+// RUN: qss-compiler -X=qasm --emit=mlir %s | FileCheck %s --match-full-lines --check-prefix MLIR
+
+gate h q {
+    U(1.57079632679, 0.0, 3.14159265359) q;
+}
+
+qubit $0;
+int n = 1;
+
+bit is_excited;
+
+// AST-PRETTY: WhileStatement(condition=BinaryOpNode(type=ASTOpTypeCompNeq, left=IdentifierNode(name=n, bits=32), right=IntNode(signed=true, value=0, bits=32))
+// MLIR: scf.while : () -> () {
+// MLIR:     %2 = quir.use_variable @n : i32
+// MLIR:     %c0_i32_0 = arith.constant 0 : i32
+// MLIR:     %3 = arith.cmpi ne, %2, %c0_i32_0 : i32
+// MLIR:     scf.condition(%3)
+// MLIR: } do {
+while (n != 0) {
+    // AST-PRETTY: statements=
+    // AST-PRETTY: HGateOpNode(params=[], qubits=[], qcparams=[$0])
+    // MLIR: quir.call_gate @h(%0) : (!quir.qubit<1>) -> ()
+    // MLIR: %cst = constant unit
+    h $0;
+    // MLIR: %2 = quir.measure(%0) : (!quir.qubit<1>) -> i1
+    // MLIR: quir.assign_cbit_bit @is_excited<1> [0] : i1 = %2
+    // MLIR: %3 = quir.use_variable @is_excited : !quir.cbit<1>
+    // MLIR: %4 = "quir.cast"(%3) : (!quir.cbit<1>) -> i1
+    is_excited = measure $0;
+    // MLIR: scf.if %4 {
+    if (is_excited) {
+        // MLIR:     quir.call_gate @h(%0) : (!quir.qubit<1>) -> ()
+        // MLIR:     %cst_1 = constant unit
+        // MLIR: }
+        h $0;
+    }
+    // error: Binary operation ASTOpTypeSub not supported yet.
+    // n = n - 1;
+    // MLIR: %c0_i32_0 = arith.constant 0 : i32
+    // MLIR: quir.assign_variable @n : i32 = %c0_i32_0
+    n = 0;  // workaround for n = n - 1
+    // MLIR: scf.yield
+}
+// AST-PRETTY: )

test/Frontend/OpenQASM3/while-2.qasm

@@ -0,0 +1,46 @@
+OPENQASM 3.0;
+// RUN: qss-compiler -X=qasm --emit=ast-pretty %s | FileCheck %s --match-full-lines --check-prefix AST-PRETTY
+// RUN: qss-compiler -X=qasm --emit=mlir %s | FileCheck %s --match-full-lines --check-prefix MLIR
+
+gate h q {
+    U(1.57079632679, 0.0, 3.14159265359) q;
+}
+
+qubit $0;
+int n = 2;
+bit is_excited;
+
+// AST-PRETTY: WhileStatement(condition=BinaryOpNode(type=ASTOpTypeCompNeq, left=IdentifierNode(name=n, bits=32), right=IntNode(signed=true, value=0, bits=32))
+// MLIR: scf.while : () -> () {
+// MLIR:     %2 = quir.use_variable @n : i32
+// MLIR:     %c0_i32_0 = arith.constant 0 : i32
+// MLIR:     %3 = arith.cmpi ne, %2, %c0_i32_0 : i32
+// MLIR:     scf.condition(%3)
+// MLIR: } do {
+while (n != 0) {
+    // MLIR: %2 = quir.use_variable @n : i32
+    // MLIR: %c2_i32_0 = arith.constant 2 : i32
+    // MLIR: %3 = arith.cmpi eq, %2, %c2_i32_0 : i32
+    // MLIR: scf.if %3 {
+    if (n == 2) {
+        // MLIR: quir.call_gate @h(%0) : (!quir.qubit<1>) -> ()
+        // MLIR: %cst = constant unit
+        // MLIR: %c1_i32 = arith.constant 1 : i32
+        // MLIR: quir.assign_variable @n : i32 = %c1_i32
+        h $0;
+        n = 1;
+    // MLIR: } else {
+    } else {
+        // MLIR: quir.call_gate @h(%0) : (!quir.qubit<1>) -> ()
+        // MLIR: %cst = constant unit
+        // MLIR: %4 = quir.measure(%0) : (!quir.qubit<1>) -> i1
+        // MLIR: quir.assign_cbit_bit @is_excited<1> [0] : i1 = %4
+        // MLIR: %c0_i32_1 = arith.constant 0 : i32
+        // MLIR: quir.assign_variable @n : i32 = %c0_i32_1
+        h $0;
+        is_excited = measure $0;
+        n = 0;
+    }
+    // MLIR: scf.yield
+}
+// AST-PRETTY: )