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

^KT-50875 Fixed

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

@@ -0,0 +1,17 @@
+FILE: kt50875.kt
+    public abstract interface A : R|kotlin/Any| {
+        public abstract fun foo(): R|kotlin/Unit|
+
+    }
+    @R|kotlin/Suppress|(names = vararg(String(INVISIBLE_MEMBER), String(INVISIBLE_REFERENCE))) public final fun <K> checkNotNull(x: R|K?|, y: R|K|): R|@Exact K| {
+        ^checkNotNull R|<local>/x| ?: R|<local>/y|
+    }
+    public final fun test_1(any: R|kotlin/Any|, a: R|A|, nullableA: R|A?|): R|kotlin/Unit| {
+        lvar x: R|kotlin/Any| = R|<local>/any|
+        R|<local>/x| = R|/checkNotNull|<R|kotlin/Any|>(R|<local>/nullableA|, R|<local>/a|)
+        R|<local>/x|.<Unresolved name: foo>#()
+        R|<local>/x| = R|<local>/nullableA| ?: R|<local>/a|
+        R|<local>/x|.<Unresolved name: foo>#()
+        R|<local>/x| = R|<local>/nullableA| ?: ^test_1 Unit
+        R|<local>/x|.R|/A.foo|()
+    }

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
+}