CodeGen: BlockPlacement: Clear ComputedEdges between functions.

Not clearing was causing non-deterministic compiles for large files. Addresses
for MachineBasicBlocks would end up colliding and we would lay out a block that
we assumed had been pre-computed when it had not been.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@300022 91177308-0d34-0410-b5e6-96231b3b80d8