[JS IR BE] Remove conservative unit materialization

Remove UnitMaterializationLowering
Remove Unit insertion in BridgeConstruction
Fix codegen for when expressions without else branch

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/lower/UnitMaterializationLowering.kt

@@ -1,45 +0,0 @@
-/*
- * Copyright 2010-2018 JetBrains s.r.o. and Kotlin Programming Language contributors.
- * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
- */
-
-package org.jetbrains.kotlin.backend.common.lower
-
-import org.jetbrains.kotlin.backend.common.BackendContext
-import org.jetbrains.kotlin.backend.common.FileLoweringPass
-import org.jetbrains.kotlin.ir.UNDEFINED_OFFSET
-import org.jetbrains.kotlin.ir.declarations.IrFile
-import org.jetbrains.kotlin.ir.expressions.IrCall
-import org.jetbrains.kotlin.ir.expressions.IrExpression
-import org.jetbrains.kotlin.ir.expressions.impl.IrCompositeImpl
-import org.jetbrains.kotlin.ir.expressions.impl.IrGetObjectValueImpl
-import org.jetbrains.kotlin.ir.symbols.IrClassSymbol
-import org.jetbrains.kotlin.ir.types.classifierOrFail
-import org.jetbrains.kotlin.ir.types.isUnit
-import org.jetbrains.kotlin.ir.visitors.IrElementTransformerVoid
-import org.jetbrains.kotlin.ir.visitors.transformChildrenVoid
-
-class UnitMaterializationLowering(context: BackendContext) : FileLoweringPass {
-
-    private val unitType = context.irBuiltIns.unitType
-    private val unitValue
-        get() = IrGetObjectValueImpl(
-            UNDEFINED_OFFSET,
-            UNDEFINED_OFFSET,
-            unitType,
-            unitType.classifierOrFail as IrClassSymbol
-        )
-
-    override fun lower(irFile: IrFile) {
-        irFile.transformChildrenVoid(object : IrElementTransformerVoid() {
-            override fun visitCall(expression: IrCall): IrExpression {
-                expression.transformChildrenVoid(this)
-                return if (expression.type.isUnit()) expression.let {
-                    IrCompositeImpl(it.startOffset, it.endOffset, it.type, it.origin, listOf(it, unitValue))
-                }
-                else expression
-            }
-        })
-    }
-
-}