Remove dead code assignments and variable declarations.

Remove dead code assignments and variable declarations.

This pass only works on assignments where the variable is never
referenced.  There is no code flow analysis, so it can't do a better
job of avoiding redundant assignments.

For now, the optimizer only does do_dead_code_unlinked(), so it won't
trim the builtin variable list or initializers outside of the scope of
functions.  This is because we don't have the visibility into other
functions that might get linked in in order to eliminate work on
global variables.