[InstCombine] Allow sinking from unique predecessor with multiple edges

Summary: We can allow sinking if the single user block has only one unique predecessor, regardless of the number of edges. Note that a switch statement with multiple cases can have the same destination.

Reviewers: mcrosier, majnemer, spatel, reames

Subscribers: reames, mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D23722

llvm-svn: 279448