[MBP] Use profile count to compute tail dup cost if it is available

Current tail duplication in machine block placement pass uses block frequency
information in cost model. But frequency number has only relative meaning
compared to other basic blocks in the same function. A large frequency number
doesn't mean it is hot and a small frequency number doesn't mean it is cold.

To overcome this problem, this patch uses profile count in cost model if it's
available. So we can tail duplicate real hot basic blocks.

Differential Revision: https://reviews.llvm.org/D83265