Stop LiveVariables from using BasicBlocks as part of the mapping, instead

Stop LiveVariables from using BasicBlocks as part of the mapping, instead
use MachineBasicBlocks.  To do this, we traverse the Machine CFG instead of
the LLVM CFG, which is also *MUCH* more efficient by having fewer levels of
indirections and mappings.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@13301 91177308-0d34-0410-b5e6-96231b3b80d8