[NFC][CodeGen] Modify the type element of TailCalls to simplify the dupRetToEnableTailCallOpts()

Summary:
The old code can be simplified to define the element type of TailCalls as `BasicBlock` not `CallInst`. Also I use the for-range loop instead the for loop.

Reviewed By: jsji

Differential Revision: https://reviews.llvm.org/D64905

llvm-svn: 367644