[Wasm] Fix unit issues

- Materialize unit when its value is actually needed.
- Special-case Unit_getInstance return type at codegen. It should be a
  proper Unit object instead of a "void"

compiler/testData/codegen/box/unit/UnitValue.kt

@@ -1,5 +1,3 @@
-// DONT_TARGET_EXACT_BACKEND: WASM
-// WASM_MUTE_REASON: UNIT_ISSUES
 fun foo() {}
 
 fun box(): String {

compiler/testData/codegen/box/unit/closureReturnsNullableUnit.kt

@@ -1,5 +1,3 @@
-// DONT_TARGET_EXACT_BACKEND: WASM
-// WASM_MUTE_REASON: UNIT_ISSUES
 fun isNull(x: Unit?) = x == null
 
 fun box(): String {

compiler/testData/codegen/box/unit/kt3634.kt

@@ -1,5 +1,3 @@
-// DONT_TARGET_EXACT_BACKEND: WASM
-// WASM_MUTE_REASON: UNIT_ISSUES
 val c = Unit
 val d = c
 

compiler/testData/codegen/box/unit/kt4212.kt

@@ -1,5 +1,3 @@
-// DONT_TARGET_EXACT_BACKEND: WASM
-// WASM_MUTE_REASON: UNIT_ISSUES
 fun foo(): Any? = bar()
 
 fun bar() {}

compiler/testData/codegen/box/unit/kt4265.kt

@@ -1,5 +1,3 @@
-// DONT_TARGET_EXACT_BACKEND: WASM
-// WASM_MUTE_REASON: UNIT_ISSUES
 fun <T : Any, R> T.let(f: (T) -> R): R = f(this)
 
 fun box(): String {

compiler/testData/codegen/box/unit/nullableUnit.kt

@@ -1,5 +1,3 @@
-// DONT_TARGET_EXACT_BACKEND: WASM
-// WASM_MUTE_REASON: UNIT_ISSUES
 fun isNull(x: Unit?) = x == null
 
 fun <T : Any> isNullGeneric(x: T?) = x == null

compiler/testData/codegen/box/unit/nullableUnitInWhen3.kt

@@ -1,5 +1,3 @@
-// DONT_TARGET_EXACT_BACKEND: WASM
-// WASM_MUTE_REASON: UNIT_ISSUES
 fun foo() {}
 
 fun box(): String {

compiler/testData/codegen/box/unit/unitClassObject.kt

@@ -1,5 +1,3 @@
-// DONT_TARGET_EXACT_BACKEND: WASM
-// WASM_MUTE_REASON: UNIT_ISSUES
 fun box(): String {
     Unit