NI: Support forking inference with heuristics

Mostly, it only affects FIR

It partially allows to consider several variance of constraints like
A<Int> & A<T> <: A<X_var> that are mostly brought by smart casts

^KT-49542 Fixed
^KT-50489 Relates

core/util.runtime/src/org/jetbrains/kotlin/utils/addToStdlib.kt

@@ -235,3 +235,5 @@ inline fun <T, U, K, V> List<T>.flatGroupBy(
     }
     return result
 }
+
+fun <E> MutableList<E>.popLast(): E = removeAt(lastIndex)