[SimpleLoopUnswitch] Don't consider unswitching `switch` insructions with one unique successor

Only instructions with two or more unique successors should be considered for unswitching.

Patch Author: Daniil Suchkov.

Reviewers: reames, asbirlea, skatkov
Reviewed By: skatkov
Subscribers: hiraditya, llvm-commits
Differential Revision: https://reviews.llvm.org/D64404

llvm-svn: 365611