[MergeICmp] We can discard initial blocks that do other work

Summary:
 We can discard initial blocks that do other work
We do not need to limit ourselves to just the first block in the chain.

Reviewers: courbet, davide

Reviewed By: courbet

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D44029

llvm-svn: 326698