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/fir/analysis-tests/testData/resolve/arguments/fieldPlusAssign.fir.txt

@@ -6,6 +6,6 @@ FILE: fieldPlusAssign.kt
         }
     public final val y: R|kotlin/Int| = Int(1)
         public get(): R|kotlin/Int| {
-            <Variable expected># = F|/y|.R|kotlin/Int.plus|(Int(1))
+            <Re-assigning a val variable># = F|/y|.R|kotlin/Int.plus|(Int(1))
             ^ Int(1)
         }

compiler/fir/analysis-tests/testData/resolve/arguments/fieldPlusAssign.kt

@@ -5,6 +5,6 @@ var x: Int = 1
 
 val y: Int = 1
     get() {
-        <!VARIABLE_EXPECTED!>field<!> += 1
+        <!VAL_REASSIGNMENT_VIA_BACKING_FIELD_ERROR!>field<!> += 1
         return 1
     }