[Thumbv8] Fix the value of BLXOperandIndex of isV8EligibleForIT

Originally, BLX was passed as operand #0 in MachineInstr and as operand
#2 in MCInst. But now, it's operand #2 in both cases.

This patch also removes unnecessary FileCheck in the test case added by r199127.

llvm-svn: 199928