[SelectionDAG] Use a union of bitfield structs for SDNode::SubclassData.

Summary:
This greatly simplifies our handling of SDNode::SubclassData.

NFC, hopefully.  :)

See discussion in D23035 for discussion about the design API of these
bitfields.

Reviewers: chandlerc

Subscribers: llvm-commits, rnk

Differential Revision: https://reviews.llvm.org/D23036

llvm-svn: 279537