[SimplifyCFG] Cleanup redundant conditions [NFC].

[SimplifyCFG] Cleanup redundant conditions [NFC].

Summary:
Since the for loop iterates over BB's predecessors, the branch conditions found must have BB as one of the successors.
For an unconditional branch the successor must be BB, added `assert`.
For a conditional branch, one of the two successors must be BB, simplify `else if` to `else` and `assert`.
Sink common instructions outside the if/else block.

Reviewers: sanjoy.google

Subscribers: jlebar, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65596

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@367699 91177308-0d34-0410-b5e6-96231b3b80d8