i965: Add an assertion that writemask != NULL for non-ARFs.

We've observed GPU hangs on Ivybridge from the following instruction:

mov(8) g115<1>.F 0D { align16 WE_normal NoDDChk 1Q };

There should be no reason to ever set the writemask on a destination
register to zero, except for perhaps the ARF NULL register.

This patch adds an assertion to enforce this for non-ARF registers.
Excluding ARFs is conservative yet should still catch the majority
of mistakes.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>

src/mesa/drivers/dri/i965/brw_eu_emit.c

@@ -126,6 +126,8 @@ brw_set_dest(struct brw_compile *p, struct brw_instruction *insn,
       else {
 	 insn->bits1.da16.dest_subreg_nr = dest.subnr / 16;
 	 insn->bits1.da16.dest_writemask = dest.dw1.bits.writemask;
+         assert(dest.dw1.bits.writemask != 0 ||
+                dest.file == BRW_ARCHITECTURE_REGISTER_FILE);
 	 /* From the Ivybridge PRM, Vol 4, Part 3, Section 5.2.4.1:
 	  *    Although Dst.HorzStride is a don't care for Align16, HW needs
 	  *    this to be programmed as "01".