pass is required to paper over problems in the code generator (primarily
live variables and its clients) which doesn't really have any well defined
semantics for unreachable code.

The proper solution to this problem is to have instruction selectors not
select blocks that are unreachable.  Until we have a instruction selection
framework available for use, however, we can't expect all instruction
selector writers to do this.  Until then, this pass should be used.

llvm-svn: 14563