[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@279537 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed