Reland "b19ec1eb [BPI] Improve unreachable/ColdCall heurstics to handle loops."

Summary: b19ec1eb has been reverted because of the test failures
with PowerPC targets. This patch addresses the issues from the previous
commit.

Test Plan: ninja check-all. Confirmed that CodeGen/PowerPC/pr36292.ll
and CodeGen/PowerPC/sms-cpy-1.ll pass

Subscribers: llvm-commits