[MergeICmp] Split blocks that do other work.

Summary:
We do not try to move the instructions and split the block till we
know the blocks can be split, i.e. BCE-cmp-insts can be separated from
non-BCE-cmp-insts.

Reviewers: davide, courbet

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D44443

llvm-svn: 329564