Use get_thread_regcache instead of get_current_regcache in post_create_inferior
    
    In post_create_inferior, we get the current thread using the
    inferior_thread function and store it in `thr`.  We then call
    get_current_regcache immediately after, which does:
    
      return get_thread_regcache (inferior_thread ());
    
    This patch makes post_create_inferior use get_thread_regcache, passing
    `thr`, saving an unnecessary inferior_thread call.
    
    gdb/ChangeLog:
    
    	* infcmd.c (post_create_inferior): Use get_thread_regcache
    	instead of get_current_regcache.