iRule to Rewrite User Agent Header

try this code which does the same as your irule (optimized):

when HTTP_REQUEST { 
     Rewrite the User-Agent header value to show up supported browser 
    switch -glob -- [string toupper [HTTP::header User-Agent]] {
        "*MSIE [6789]*" {
            HTTP::header replace "User-Agent" "Mozilla/5.0 (Windows NT 10.0; WOW64; Trident/7.0; rv:11.0) like Gecko" 
            log local0. "[HTTP::header User-Agent]" 
        }
        default {
        }
    }
}