Replace CAS loop with single TAS in ProcArrayGroupClearXid()

Replace CAS loop with single TAS in ProcArrayGroupClearXid()

Single pg_atomic_exchange_u32() is expected to be faster than loop of
pg_atomic_compare_exchange_u32().  Also, it would be consistent with
clog group update code.

Discussion: https://postgr.es/m/CAPpHfdtxLsC-bqfxFcHswZ91OxXcZVNDBBVfg9tAWU0jvn1tQA%40mail.gmail.com
Reviewed-by: Amit Kapila