i965: let the if_stack just store the instruction index

i965: let the if_stack just store the instruction index

If dynamic instruction store size is enabled, while after
the brw_IF/ELSE() and before the brw_ENDIF() function, the
eu instruction store base address(p->store) may change.

Thus let if_stack just store the instruction index. This is
somehow more flexible and safe than store the instruction
memory address.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>