[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.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199928 91177308-0d34-0410-b5e6-96231b3b80d8