i965/vs: Fix incorrect subscript when resetting copy propagation records.

In this code, 'i' loops over the number of virtual GRFs, while 'j' loops
over the number of vector components (0 <= j <= 3).

It can't possibly be correct to see if bit 'i' is set in the destination
writemask, as it will have values much larger than 3.  Clearly this is
supposed to be 'j'.

Found by inspection.

Tested-by: Matt Turner <mattst88@gmail.com>
Tested-by: Christopher James Halse Rogers <christopher.halse.rogers@canonical.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/i965/brw_vec4_copy_propagation.cpp

@@ -323,7 +323,7 @@ vec4_visitor::opt_copy_propagation()
 
 	    for (int i = 0; i < virtual_grf_reg_count; i++) {
 	       for (int j = 0; j < 4; j++) {
-		  if (inst->dst.writemask & (1 << i) &&
+		  if (inst->dst.writemask & (1 << j) &&
 		      cur_value[i][j] &&
 		      cur_value[i][j]->file == GRF &&
 		      cur_value[i][j]->reg == inst->dst.reg &&