[X86][TableGen] Fix the mnemonic table for CMPCCXADD

The mnemonic of CMPCCXADD is `cmp${cond}xadd` and the condition code
is in the middle of mnemonic. When generating the function name for
CMPCCXADD, the substring `xadd` should be kept.

Before this patch, the name is `isCMPCC`. After this patch, the name
is `isCMPCCXADD`.