FIR: Fix inference for case of non-nullable RHS of elvis

Some existing tests start failing after previous commits adding @Exact
attribute to `?:`

They have a form:
var x: String? = nullable()

if (x == null) {
   x = nullable() ?: "" // considering @Exact the whole elvis is inferred to nullable from expect type
}

x.length // should be smart cast

compiler/fir/analysis-tests/testData/resolveWithStdlib/inference/ifElvisReturn.fir.txt

@@ -14,7 +14,7 @@ FILE: ifElvisReturn.kt
                 R|<local>/b|
             }
             else ->  {
-                R|/materialize|<R|kotlin/Nothing|>() ?: ^foo Unit
+                R|/materialize|<R|B|>() ?: ^foo Unit
             }
         }