cmd/8c: fix store to complex uint64 ptr
cmd/8c: fix store to complex uint64 ptr

Assignment of a computed uint64 value to an
address derived with a function call was executing
the call after computing the value, which trashed
the value (held in registers).

long long *f(void) { return 0; }
void g(int x, int y) {
        *f() = (long long)x | (long long)y<<32;
}

Before:

(x.c:3)	TEXT	g+0(SB),(gok(71))
...
(x.c:4)	ORL	AX,DX
(x.c:4)	ORL	CX,BX
(x.c:4)	CALL	,f+0(SB)
(x.c:4)	MOVL	DX,(AX)
(x.c:4)	MOVL	BX,4(AX)

After:
(x.c:3)	TEXT	g+0(SB),(gok(71))
(x.c:4)	CALL	,f+0(SB)
...
(x.c:4)	ORL	CX,BX
(x.c:4)	ORL	DX,BP
(x.c:4)	MOVL	BX,(AX)
(x.c:4)	MOVL	BP,4(AX)

Fixes issue 3501.

R=ken2
CC=golang-dev
http://codereview.appspot.com/5998043

sanjosh/sanjos100-tipc,d0f/go-zh,d0f/go-zh,Triskite/willstone-goclone,rflanagan/reginaldflanagan-project1,webfd/go-zh,mhennings/marcohennings-go,mhennings/marcohennings-go,Triskite/willstone-goclone,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,rdp/rogerpack2005-golang,d0f/go-zh,Triskite/willstone-goclone,sanjosh/sanjos100-tipc,Triskite/willstone-goclone,d0f/go-zh,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,rflanagan/reginaldflanagan-project1,webfd/go-zh,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,scirelli/scirelli-go,rdp/rogerpack2005-golang,d0f/go-zh,rdp/rogerpack2005-golang,scirelli/scirelli-go,Triskite/willstone-goclone,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,d0f/go-zh,webfd/go-zh,Triskite/willstone-goclone,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,bryanxu/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,d0f/go-zh,mhennings/marcohennings-go,bryanxu/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,d0f/go-zh,Triskite/willstone-goclone,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,bryanxu/go-zh,bryanxu/go-zh,scirelli/scirelli-go,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,bryanxu/go-zh,webfd/go-zh,Triskite/willstone-goclone