Large code model support for PowerPC.

Large code model is identical to medium code model except that the
addis/addi sequence for "local" accesses is never used.  All accesses
use the addis/ld sequence.

The coding changes are straightforward; most of the patch is taken up
with creating variants of the medium model tests for large model.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175767 91177308-0d34-0410-b5e6-96231b3b80d8