When a temporary is introduced to break a cycle, we neglected to update the predecessor count for one of the operands, leading to a possible assertion failure. This problem isn't currently seen in master, but it arises when we enable register aliases, as in https://codereview.chromium.org/1427973003/ . No changes are seen in spec2k code generation as a result of this fix. A consistency check is added to help find future problems like this. Also, refactored iteration over the Phi descriptor array to use range-based for loops and avoid directly indexing the array. BUG= none R=jpp@chromium.org Review URL: https://codereview.chromium.org/1435543002 .