Revert r254348: "Replace all weight-based interfaces in MBB with...

Revert r254348: "Replace all weight-based interfaces in MBB with probability-based interfaces, and update all uses of old interfaces."

and the follow-up r254356: "Fix a bug in MachineBlockPlacement that may cause assertion failure during BranchProbability construction."

Asserts were firing in Chromium builds. See PR25687.

llvm-svn: 254366