This patch arranges for PHI nodes to be propagated into debug stmts upon
release, if possible.  This improves debug info quality a bit, for
without this change we'd end up resetting the debug stmt.

It's not clear to me how this helped (if at all) graphite failures with
VTA, but it helped retain debug annotations that enabled me to validate
the fix for PR 41926 in a C testcase.  But that's something for another
patch.

Bootstrapped on x86_64-linux-gnu; regstrapping on that and
ia64-linux-gnu.  Ok to install?