[PowerPC] Merge/rename PPC fixup types

[PowerPC] Merge/rename PPC fixup types

Now that fixup_ppc_ha16 and fixup_ppc_lo16 are being treated exactly
the same everywhere, it no longer makes sense to have two fixup types.

This patch merges them both into a single type fixup_ppc_half16,
and renames fixup_ppc_lo16_ds to fixup_ppc_half16ds for consistency.
(The half16 and half16ds names are taken from the description of
relocation types in the PowerPC ABI.)

No change in code generation expected.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182092 91177308-0d34-0410-b5e6-96231b3b80d8