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

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>