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/UninitializedOrReassignedVariables.fir.kt

@@ -89,18 +89,18 @@ fun t3() {
 
 fun t4() {
     val x = 1
-    <!VARIABLE_EXPECTED!>x<!> += 2
+    <!VAL_REASSIGNMENT!>x<!> += 2
     val y = 3
-    <!VARIABLE_EXPECTED!>y<!> *= 4
+    <!VAL_REASSIGNMENT!>y<!> *= 4
     var z = 5
     z -= y
 }
 
 fun t5() {
     for (i in 0..2) {
-        <!VARIABLE_EXPECTED!>i<!> += 1
+        <!VAL_REASSIGNMENT!>i<!> += 1
         fun t5() {
-            <!VARIABLE_EXPECTED!>i<!> += 3
+            <!VAL_REASSIGNMENT!>i<!> += 3
         }
     }
 }
@@ -213,7 +213,7 @@ class LocalValsVsProperties(val a: Int, w: Int) : Open(a, w) {
     var xx = w
     var yy : Int
     init {
-        <!VARIABLE_EXPECTED!>w<!> += 1
+        <!VAL_REASSIGNMENT!>w<!> += 1
         yy = w
     }
 }

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"
         }
     }
 }

compiler/testData/diagnostics/tests/controlFlowAnalysis/kt1571.fir.kt

@@ -31,7 +31,7 @@ fun box() : String {
     if (c1 != 1) {
         return "2"
     }
-    <!VARIABLE_EXPECTED!>a<!> *= 3 // a = a * 3, shouldn't be able to do this on val
+    <!VAL_REASSIGNMENT!>a<!> *= 3 // a = a * 3, shouldn't be able to do this on val
     if (c0 != 2) {
         return "3"
     }