phi/mergemem transformations

[John Rose]

It's actually this delta:
  D 1.174 00/06/19 00:47:54 jrose 434 431
  Phi nodes of type Memory now record their alias type also.
  Add factory methods PhiNode::make/make_blank/slice_memory.
  Teach PhiNode::Ideal to split phis through memory merges.
  Add manifest constants regarding Phi/Region layout.
  Add PhiNode::region() accessor, degrade_to_copy operation.
  Adjust PhiNode::Ideal logic to split Phis through MergeMems.
  Make CreateExNode::bottom_type be accurate.

I don't remember the problem, nor why it was "temporary".

I suppose I ran into optimizer loops where a degenerate phi nest could  
cause this split logic to repeat forever.

Perhaps our eager dead code elimination makes this a thing of the past.

-- John