freedreno/a3xx/compiler: fix p0 (kill, etc)

Don't assert (debug builds) or assign random uninitialized value for
predicate register (p0).. that screws up kill, etc.

Signed-off-by: Rob Clark <robclark@freedesktop.org>

src/gallium/drivers/freedreno/a3xx/ir3_ra.c

@@ -576,7 +576,8 @@ static void ir3_instr_ra(struct ir3_ra_ctx *ctx,
 		dst->flags &= ~IR3_REG_ADDR;
 		num = regid(REG_A0, 0) | REG_HALF;
 	} else {
-		assert(0);
+		/* predicate register (p0).. etc */
+		return;
 	}
 
 	ra_assign(ctx, instr, num);