* Stop using GenTree::ReplaceWith
* GenTreeRetExpr now directly contains its substitution members, which
  is a tree either from the inline candidate, a local (due to spill
  temps) or the call itself (due to inliner failure). In the former
  case, it also contains the BasicBlock the child tree comes from to
  ensure that its flags can be propagated once and for all during
  substitution.
* Make various members strongly typed, e.g.
  GenTreeRetExpr::gtInlineCandidate is now a GenTreeCall*,
  InlineCandidateInfo::retExpr is now a GenTreeRetExpr*. This is
  possible since we are no longer using GenTree::ReplaceWith.

When we fail to inline, the substitution will be the call itself, so we
should propagate flags from the basic block containing the call. In
particular this can be a problem with RET_EXPR chains.