SelectionDAG: Use correct pointer size when lowering function arguments v2

This adds minimal support to the SelectionDAG for handling address spaces
with different pointer sizes.  The SelectionDAG should now correctly
lower pointer function arguments to the correct size as well as generate
the correct code when lowering getelementptr.

This patch also updates the R600 DataLayout to use 32-bit pointers for
the local address space.

v2:
  - Add more helper functions to TargetLoweringBase
  - Use CHECK-LABEL for tests

llvm-svn: 189221