[Indic] Don't clear the mask for Reph

This was removing the mandatory global 1 bit in the mask and hence
disabling GPOS for Reph!

src/hb-ot-shape-complex-indic.cc

@@ -382,7 +382,7 @@ initial_reordering_consonant_syllable (const hb_ot_map_t *map, hb_buffer_t *buff
 
     /* Reph */
     if (has_reph)
-      info[start].mask = mask_array[RPHF];
+      info[start].mask |= mask_array[RPHF];
 
     /* Pre-base */
     mask = mask_array[HALF] | mask_array[AKHN] | mask_array[CJCT];