[FIR] Implement new bound smartcast algorithm

#KT-36055 Fixed

compiler/testData/diagnostics/testsWithStdLib/smartcasts/letAlwaysChangesToNotNull.fir.kt

@@ -3,5 +3,5 @@
 fun foo(y: String) {
     var x: String? = null
     y.let { x = it }
-    x.<!INAPPLICABLE_CANDIDATE!>length<!> // Smart cast is not possible
+    x.length // Smart cast is not possible
 }

compiler/testData/diagnostics/testsWithStdLib/smartcasts/letMergeNotNull.fir.kt

@@ -4,6 +4,6 @@ fun foo(y: String?) {
     var x: String? = null
     if (x != null) {
         y?.let { x = it }
-        x.<!INAPPLICABLE_CANDIDATE!>length<!> // not-null or not-null
+        x.length // not-null or not-null
     }
 }