[Raw FIR] Build if - else if - else as two nested whens

This is necessary for inference to work like in K1 because we only
add equality constraints from expected types on top-level `when`, not
on nested ones.

#KT-65882

compiler/testData/diagnostics/tests/inference/nestedIfWithExpectedType.kt

@@ -3,7 +3,15 @@ interface Promise<T>
 
 fun <T> materializePromise(): Promise<T> = TODO()
 
-fun foo(x: Int, p1: Promise<Any?>, p2: Promise<Nothing?>): Promise<*> {
+fun foo0(x: Int, p2: Promise<Nothing?>): Promise<*> {
+    return if (x == 3) {
+        <!NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER!>materializePromise<!>()
+    } else {
+        p2
+    }
+}
+
+fun foo1(x: Int, p1: Promise<Any?>, p2: Promise<Nothing?>): Promise<*> {
     return if (x == 0) {
         p1
     } else {
@@ -15,10 +23,12 @@ fun foo(x: Int, p1: Promise<Any?>, p2: Promise<Nothing?>): Promise<*> {
     }
 }
 
-fun foo2(x: Int, p2: Promise<Nothing?>): Promise<*> {
-    return if (x == 3) {
-        <!NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER!>materializePromise<!>()
+fun foo2(x: Int, p1: Promise<Any?>): Promise<*> {
+    return if (x == 0) {
+        p1
+    } else if (x == 2) {
+        materializePromise()
     } else {
-        p2
+        p1
     }
-}
+}