[CodeGen] Allow partial tail duplication in Machine Block Placement.

Summary: A count profile may affect tail duplication's heuristic causing a block to be duplicated in only a part of its predecessors. This is not allowed in the Machine Block Placement pass where an assert will go off. I'm removing the assert and making the optimization bail out when such case happens.

Reviewers: wenlei, davidxl, Carrot

Reviewed By: Carrot

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77748