Belinda Carlisle is on the verge of signing a multiple-record, seven-figure recording contract with PolyGram Records, says a source close to the former Go-Go's lead singer. Carlisle made her first solo album for I.R.S. Records last year--but although that LP included the hit "Mad About You," I.R.S. forgot to renew her contract and she became a free agent. The PolyGram deal should be completed by the end of the week. I.R.S., which took Carlisle to court in an unsuccessful attempt to retain her services, declined to comment.