Refactor X86 symbol access classification.

This refactors the logic in X86 to avoid code duplication. It also
splits it in two steps: it first decides if a symbol is local to the DSO
and then uses that information to decide how to access it.

The first part is implemented by shouldAssumeDSOLocal. It is not in any
way specific to X86. In a followup patch I intend to move it to
somewhere common and reused it in other backends.

llvm-svn: 270209