[FIR] Infer type of elvis from lhs if rhs is Nothing

^KT-50875 Fixed

compiler/fir/analysis-tests/testData/resolve/smartcasts/kt50875.kt

@@ -0,0 +1,23 @@
+// ISSUE: KT-50875
+
+interface A {
+    fun foo()
+}
+
+@Suppress("INVISIBLE_MEMBER", "INVISIBLE_REFERENCE")
+fun <K> checkNotNull(x: K?, y: K): @kotlin.internal.Exact K {
+    return x ?: y
+}
+
+fun test_1(any: Any, a: A, nullableA: A?) {
+    var x = any // x has type Any
+
+    x = checkNotNull(nullableA, a) // inferred to Any
+    x.<!UNRESOLVED_REFERENCE!>foo<!>() // no smartcast
+
+    x = nullableA ?: a // inferred to Any
+    x.<!UNRESOLVED_REFERENCE!>foo<!>() // no smartcast
+
+    x = nullableA ?: return // inferred to Any, but has A type
+    x.foo() // smartcast
+}