Removes a branch that checks for a condition that has been checked on dominators of the branch.

This introduces a new reducer that propagates the list of checked conditions (and their boolean values) through the control flow graph. If it encounters a branch checking a condition with a known value, the branch is eliminated.

The analysis relies on loops being reducible: if a condition has been checked on all paths to loop entry, then it is checked in the loop (regardless what of the conditions checked inside the loop).

The implementation is fairly naive and could be improved:

- all the operation on the condition lists could be made allocation-free when revisited.

- we could try to use a map structure rather than a linked list (to make
lookups faster).

- the merging of control flow could be changed to take into account
  conditions from non-dominating paths (as long as all paths check
  the condition).

Review URL: https://codereview.chromium.org/1376293005

Cr-Commit-Position: refs/heads/master@{#31347}