BranchRelaxation: Recompute live-ins when splitting a block

Factors out and reuses live-in computation code from BranchFolding.

Differential Revision: https://reviews.llvm.org/D27558

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@290013 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed