[CodeGen] Implement the SUnit::print() method

This method seems to have had a troubled life.  This patch proposes that it
replaces the recently added helper function dumpSUIdentifier.  This way, the
method can be used in other files using the SUnit class.

Differential revision: https://reviews.llvm.org/D28488

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@291520 91177308-0d34-0410-b5e6-96231b3b80d8