Show specific messages for some more invalid instructions, not just "syntax error" (#1679)

test/asm/invalid-instructions.asm

@@ -6,3 +6,16 @@ SECTION "invalid", ROM0[$10000]
 	ld b, [$4000]
 	bit 8, a
 	rst $40
+
+	ld bc, bc
+	ld de, hl
+	ld hl, de
+
+	ld hl, sp ; no offset!
+	; ld sp, hl is valid
+
+	ld sp, bc
+	ld bc, sp
+
+	ld af, bc
+	ld bc, af