Fixed bug on premature DCHECK.

With fail-before/pass-after test

bug=26947011

Rationale:
During BCE, the phi structure is under construction,
to be fixed by InsertPhiNodes() and carefully checked
with the SSA checker. So utilities should not overly
DCHECK on SSA consistency during the modifications.

Change-Id: Ia9df9ee5aac0c1dd2c3e3a447c730246d5e48bbb