Implement compound assignment operators **= and >>>= (#2158)

test/asm/compound-assignment.asm

@@ -11,8 +11,10 @@ println \2 ; 14
 println \2 ; 28
 {prefix}\2 /= 4
 println \2 ; 7
-{prefix}\2 %= 3
-println \2 ; 1
+{prefix}\2 %= 4
+println \2 ; 3
+{prefix}\2 **= 2
+println \2 ; 9
 {prefix}\2 |= 11
 println \2 ; 11
 {prefix}\2 ^= 12
@@ -23,6 +25,8 @@ println \2 ; 5
 println \2 ; 20
 {prefix}\2 >>= 1
 println \2 ; 10
+{prefix}\2 >>>= 1
+println \2 ; 5
 purge prefix
 endm
 

test/asm/compound-assignment.err

@@ -1,3 +1,3 @@
 error: Expected constant expression: undefined symbol `UnDeFiNeD`
-    at compound-assignment.asm(35)
+    at compound-assignment.asm(39)
 Assembly aborted with 1 error

test/asm/compound-assignment.out

@@ -4,23 +4,27 @@ $F
 $E
 $1C
 $7
-$1
+$3
+$9
 $B
 $7
 $5
 $14
 $A
+$5
 redef r:
 $A
 $F
 $E
 $1C
 $7
-$1
+$3
+$9
 $B
 $7
 $5
 $14
 $A
+$5
 $64
 $0