Add -verify-misched option.

This verifies live intervals both before and after scheduling. It's
useful for anyone hacking on live interval update.

Note that we don't yet pass verification all the time. We don't yet
handle updating nonallocatable live intervals perfectly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176685 91177308-0d34-0410-b5e6-96231b3b80d8