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 #3501.
R=ken2
CC=golang-dev
https://golang.org/cl/
5998043