code emission location into the base class, instead of being in the derived classes.
This change means that low-level methods like emitByte/emitWord now are no longer
virtual (yaay for speed), and we now have a framework to support growable code
segments. This implements feature request #1 of PR469.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@28059 91177308-0d34-0410-b5e6-96231b3b80d8