Improve PC offset management

Basically make it always point to the instruction's first byte.
This was the behavior all whom I asked to intuitively expected.

test/asm/pc-operand.asm

@@ -0,0 +1,13 @@
+SECTION "fixed", ROM0[0]
+
+	rst @    ; rst 0
+	ld de, @ ; ld de, 1
+	bit @, h ; bit 4, h
+	db @, @  ; db 6, 7
+
+SECTION "floating", ROM0
+
+	rst @    ; rst 8
+	ld l, @  ; ld l, 9
+	dw @, @  ; dw 11, 13
+	dl @, @  ; dl 15, 19