[AIX] SelectionDAGNodes.h: Pack bit-fields that are meant to be packed

Summary:
Certain classes in the subject file are expected to provide different
views of a two-byte field as a collection of various bit-fields. On AIX,
the canonical layout of bit-fields would cause these classes to span
four bytes. Applying the `pack` pragma for compilers that employ the AIX
canonical layout allows these classes to fit within the expected two
bytes. In the future, the pragma would also likely need to be applied
when building with Clang on AIX.

Reviewers: xingxue, sfertile, jasonliu

Reviewed By: xingxue

Subscribers: jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60164

llvm-svn: 357661