CFG (Control Flow Graph): first iteration

Create the list of basic blocks for a particular method,
as an additional data structure on top of the method code IR.

We support two flavors of basic blocks:

compact = considers only non-exceptional control flow
verbose = models everyting, including EH flow

Bug: 37429130

Test: bazel test :dexter_tests

Change-Id: Ibf3f549bd192b9283ab2b2bafebcfa4a798a654f