AMDGPU/SI: Refactor fixup handling for constant addrspace variables

Summary:
We now use a standard fixup type applying the pc-relative address of
constant address space variables, and we have the GlobalAddress lowering
code add the required 4 byte offset to the global address rather than
doing it as part of the fixup.

This refactoring will make it easier to use the same code for global
address space variables and also simplifies the code.

Re-commit this after fixing a bug where we were trying to use a
reference to a Triple object that had already been destroyed.

Reviewers: arsenm, kzhuravl

Subscribers: arsenm, kzhuravl, llvm-commits

Differential Revision: http://reviews.llvm.org/D21154

llvm-svn: 272705