ART: Fix single-preheader transformation.

Original implementation of "Make sure the loop has only one
pre-header" had an assumption that the header had no phi
functions since loops with multiple preheaders now only may exist
during graph building before ssa construction; all of the
optimizations preserve the single-preheader invariant. This code is
used by DCE; DCE was called multiple times but after graph building
preheader transformation was never executed. However if someone
introduces a optimization which might not keep the invariant
(e.g. loop peeling) the data flow adjustments must be performed.

Test: loop_optimization_test.cc
Test: test-art-target, test-art-host
Change-Id: I88bb0aad2dd5241addef7fe9cda474a6868bf532