Assignment operations test fix:

x is a byte, x += 2 translated to x = x+2, but type of (x+2) currently
is Int, so it's not compiled because Int can't be stored in Byte.

Maybe we need to think about what should be a result of sum of to
Bytes, but currently it's still Int

compiler/testData/codegen/boxWithStdlib/regressions/kt344.kt

@@ -92,7 +92,7 @@ fun t6() : Boolean {
     val y = x + 22
     val foo = {
         x = (x + 20.toByte() + y).toByte()
-        x += 2
+        x = (x + 2).toByte()
         x--
         Unit
     }