FIR checkers: report VAL_REASSIGNMENT for assignment operators

Currently VAL_REASSIGNMENT are only reported on direct assignments.
Reassignments in the form of, for example, `+=` are reported as
`VARIABLE_EXPECTED`, which differs from FE1.0.

compiler/testData/diagnostics/tests/controlFlowAnalysis/fieldInitialization.fir.kt

@@ -68,16 +68,16 @@ class Test1 {
 
     init {
         inlineMe {
-            <!VARIABLE_EXPECTED!>a<!> += "allowed"
+            <!VAL_REASSIGNMENT!>a<!> += "allowed"
         }
         crossinlineMe {
-            <!VARIABLE_EXPECTED!>b<!> += "not allowed"
+            <!VAL_REASSIGNMENT!>b<!> += "not allowed"
         }
         noinlineMe {
-            <!VARIABLE_EXPECTED!>c<!> += "not allowed"
+            <!VAL_REASSIGNMENT!>c<!> += "not allowed"
         }
         notinline {
-            <!VARIABLE_EXPECTED!>d<!> += "not allowed"
+            <!VAL_REASSIGNMENT!>d<!> += "not allowed"
         }
     }
 }