[RS4GC] Cleanup meetBDVState. NFC.

meetBDVState looks pretty difficult to read and follow.
This is purely NFC but doing several things:

1) Combine meet and meetBDVState
2) Move the function to be a member of BDVState
3) Make BDVState be a mutable object
4) Convert switch to sequence of ifs
5) Adds comments.

Reviewers: reames, dantrushin
Reviewed By: reames
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D99064