[cc65] Literal cast to pointer generating unnecessary code with [].

I ran into an interesting case today. This is code produced by the
latest snapshot (2.13.9.20121016). This is with -O, but the same basic
problem appears without it.

;
; ((unsigned char*)0)[0x1234] = 5;
;
	jsr     decsp3
	jsr     push0
	lda     #$05
	ldy     #$01
	clc
	pha
	lda     #$12
	adc     (sp),y
	sta     (sp),y
	pla
	ldy     #$34
	jsr     staspidx
;
; *(unsigned char*)(0 + 0x1234) = 5;
;
	lda     #$05
	sta     $1234

Specifically, in the former case it ends up doing all the work
necessary to use indirect indexed addressing for the STA, instead of
just using an absolute address. I presume the compiler thinks of
((unsigned char*)0) as a variable pointer, rather than a fixed address
that should be known at compile or link time? Using a global array
seems to generate the compact absolute address STA that is expected.
Similarly I could cast it to a struct * and access that struct's
members and it still generates an absolute address. The problem only
seems to be when I use [] with the cast pointer.

-- Brad Smith
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