Make if statements return their completion values (#2739)

This Pull Request changes the following:

- Adjust if statement execution to leave their resulting values on the stack when needed.

boa_engine/src/bytecompiler/statement/if.rs

@@ -2,11 +2,11 @@ use crate::bytecompiler::ByteCompiler;
 use boa_ast::statement::If;
 
 impl ByteCompiler<'_, '_> {
-    pub(crate) fn compile_if(&mut self, node: &If, configurable_globals: bool) {
+    pub(crate) fn compile_if(&mut self, node: &If, use_expr: bool, configurable_globals: bool) {
         self.compile_expr(node.cond(), true);
         let jelse = self.jump_if_false();
 
-        self.compile_stmt(node.body(), false, configurable_globals);
+        self.compile_stmt(node.body(), use_expr, configurable_globals);
 
         match node.else_node() {
             None => {
@@ -15,7 +15,7 @@ impl ByteCompiler<'_, '_> {
             Some(else_body) => {
                 let exit = self.jump();
                 self.patch_jump(jelse);
-                self.compile_stmt(else_body, false, configurable_globals);
+                self.compile_stmt(else_body, use_expr, configurable_globals);
                 self.patch_jump(exit);
             }
         }

boa_engine/src/bytecompiler/statement/mod.rs

@@ -17,7 +17,7 @@ impl ByteCompiler<'_, '_> {
     pub fn compile_stmt(&mut self, node: &Statement, use_expr: bool, configurable_globals: bool) {
         match node {
             Statement::Var(var) => self.compile_var_decl(var),
-            Statement::If(node) => self.compile_if(node, configurable_globals),
+            Statement::If(node) => self.compile_if(node, use_expr, configurable_globals),
             Statement::ForLoop(for_loop) => {
                 self.compile_for_loop(for_loop, None, configurable_globals);
             }