[LoopSink] Do not sink instructions into non-cold blocks

Summary: This fixes PR39570.

Reviewers: danielcdh, rnk, bkramer

Reviewed By: rnk

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D54181

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346337 91177308-0d34-0410-b5e6-96231b3b80d8