i965/fs: Explicitly disallow CSE on predicated instructions.

i965/fs: Explicitly disallow CSE on predicated instructions.

The existing inst->is_partial_write() already disallows predicated
instructions, so this has no functional change.  However, it's worth
doing explicitly since the CSE pass does not consider the flag register.
This means it could blindly factor out operations that use the same
sources, but which have different condition codes set.

This prevents a regression in the next commit.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>