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/LValueAssignment.fir.kt

@@ -103,7 +103,7 @@ class Test() {
         a += 34
         (l@ a) += 34
 
-        <!VARIABLE_EXPECTED!>b<!> += 34
+        <!VAL_REASSIGNMENT!>b<!> += 34
 
         a++
         (l@ a)++
@@ -113,9 +113,9 @@ class Test() {
     fun testVariables1() {
         val b: Int = 34
 
-        (l@ <!VARIABLE_EXPECTED!>b<!>) += 34
+        (l@ <!VAL_REASSIGNMENT!>b<!>) += 34
         //repeat for b
-        (<!VARIABLE_EXPECTED!>b<!>) += 3
+        (<!VAL_REASSIGNMENT!>b<!>) += 3
     }
 
     fun testArrays(a: Array<Int>, ab: Ab) {