[FIR] Lambda reassigned arguments should not have contracts applied

When applying function contracts to arguments, make sure the argument
variable hasn't been reassigned within a lambda. Contracts can only be
applied to unchanged variables, otherwise outdated type statements could
be added to the variable.

^KT-63151 Fixed

compiler/testData/diagnostics/tests/smartCasts/variables/reassignedInRhs.fir.kt

@@ -46,6 +46,6 @@ fun safeCall() {
 fun contractFunction() {
     var x: String? = ""
     if (isNotNullAlsoCall(x) { x = null }) {
-        x.length // bad (#2 == true => x#0 != null; but this is x#1 = null)
+        x<!UNSAFE_CALL!>.<!>length // bad (#2 == true => x#0 != null; but this is x#1 = null)
     }
 }