[InstCombine] Allow sinking from unique predecessor with multiple edges

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@279448 91177308-0d34-0410-b5e6-96231b3b80d8