PredicateInfo: Support switch statements

Summary:
Depends on D29606 and D29682

Makes us pass GVN's edge.ll (we also will pass a few other testcases
they just need cleaning up).

Thoughts on the Predicate* hiearchy of classes especially welcome :)
(it's not clear to me how best to organize it, and currently, the getBlock* seems ... uglier than maybe wasting a field somewhere or something).

Reviewers: davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29747

llvm-svn: 295889