fix PR68453: recursively add phi nodes to merge points

fix PR68453: recursively add phi nodes to merge points

The patch adds close phi nodes to every outer loop exit, and to every loop
guard.  For loop guards it computes an initial value that determines where we
stop inserting phi nodes.  When the initial value is a constant, the initial
value is considered to be defined in the entry of the code gen region.

Co-Authored-By: Sebastian Pop <s.pop@samsung.com>
From-SVN: r230918