The only clients of the slot calculator are now the asmwriter and bcwriter.

The only clients of the slot calculator are now the asmwriter and bcwriter.
Since this really only makes sense for these two, change hte instance variable
to reflect whether we are writing a bytecode file or not.  This makes it
reasonable to add bcwriter specific stuff to it as necessary.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@10837 91177308-0d34-0410-b5e6-96231b3b80d8