Add loop- and phi-related checks in the optimizing compiler.

- Ensure the pre-header block is first in the list of
  predecessors of a loop header.
- Ensure the loop header has only two predecessors and that
  only the second one is the back edge.
- Ensure there is only one back edge per loop.
- Ensure the first input of a phi is not itself.
- Ensure the number of phi inputs is the same as the number
  of its predecessors.
- Ensure phi input at index I either comes from the Ith
  predecessor or from a block that dominates this
  predecessor.

Change-Id: I4db5c68cfbc9b74d2d03125753d0143ece625378