[NFC] Split logic of ImplicitControlFlowTracking to allow generalization

We have a class `ImplicitControlFlowTracking` which allows us to keep track of
instructions that can abnormally exit and answer queries like "whether or not
there is side-exiting instruction above this instruction in its block".

We may want to have the similar tracking for other types of "special" instructions,
for example instructions that write memory.

This patch separates ImplicitControlFlowTracking into two classes, isolating all
general logic not related to implicit control flow into its parent class. We can
later make another child of this class to keep track of instructions that write
memory.

The motivation for that is that we want to make these checks efficiently in the
patch https://reviews.llvm.org/D50891.

NOTE: The naming of the parent class is not super cool, but the other options we
have are hardly better. Please feel free to rename it as NFC if you think you've
found a more informative name for it.

Differential Revision: https://reviews.llvm.org/D50954
Reviewed By: fedor.sergeev

llvm-svn: 340728