i965/fs: Avoid inappropriate optimization with regs_written > 1.

Right now we don't have anything with regs_written() > 1 and !inst->mlen,
but that's about to change.

NOTE: This is a candidate for the 9.1 branch.
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit bc0e1591f6)

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

@@ -2091,6 +2091,12 @@ fs_visitor::compute_to_mrf()
 	       break;
 	    }
 
+            /* Things returning more than one register would need us to
+             * understand coalescing out more than one MOV at a time.
+             */
+            if (scan_inst->regs_written() > 1)
+               break;
+
 	    /* SEND instructions can't have MRF as a destination. */
 	    if (scan_inst->mlen)
 	       break;