FIR DFA: do not generate nodes (and flows) for contracts on calls

The code was already duplicated between FirDataFlowAnalyzer and
FirReturnsImpliesAnalyzer, so might as well use the latter to slightly
speed up the former.

compiler/fir/analysis-tests/testData/resolveWithStdlib/contracts/fromSource/good/returnsImplies/propertyAccessors.kt

@@ -12,10 +12,10 @@ var Any?.isNotNull: Boolean
         return this != null
     }
     set(value) {
-        contract {
+        <!WRONG_IMPLIES_CONDITION!>contract {
             returns() implies (this@isNotNull != null)
             <!ERROR_IN_CONTRACT_DESCRIPTION!>require(<!SENSELESS_COMPARISON!>this != null<!>)<!>
-        }
+        }<!>
     }
 
 fun test_1(a: A?) {