[JS BE] Reduce temporary variable number produced during lowering

- Do not create temporary variable for pure expressions
 - Clean up unused and unreachable r-value on top level of statement block

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/ir/IrUtils.kt

@@ -14,6 +14,7 @@ import org.jetbrains.kotlin.descriptors.Modality
 import org.jetbrains.kotlin.descriptors.Visibilities
 import org.jetbrains.kotlin.descriptors.annotations.Annotations
 import org.jetbrains.kotlin.ir.IrElement
+import org.jetbrains.kotlin.ir.IrStatement
 import org.jetbrains.kotlin.ir.UNDEFINED_OFFSET
 import org.jetbrains.kotlin.ir.builders.Scope
 import org.jetbrains.kotlin.ir.declarations.*
@@ -605,4 +606,4 @@ fun copyBodyWithParametersMapping(
 }
 
 val IrSymbol.isSuspend: Boolean
-    get() = this is IrSimpleFunctionSymbol && owner.isSuspend
+    get() = this is IrSimpleFunctionSymbol && owner.isSuspend