Patch: create preheaders for RTL loop optimizer

[Dale Johannesen <dalej at apple dot com>]

The loop.c optimizer rejects all loops with multiple entry points, 
which includes
multiple predecessors of the header.  So I thought it would be a good 
idea to
force preheaders when creating loop notes.  This exposed a bug in the
preheader creation, when things are like this:
 Latch:  fallthru to header
 Header:  Latch and additional multiple predecessors
The header is split, then the latch->header edge is redirected, which 
means
creating a new block in this case, then mfb_update_loops is invoked on 
the
new block:
  Latch:  fallthru
  New block:  jump to header.2
  Preheader (old header):  multiple predecessors
  Header.2  (new header):
Then mfb_update_loops tries to use loop_father from header.2, but this 
has
not been set, so you get a crash (shows up in bootstrap on Darwin).  
Seems
simplest to copy this field in split_block.  Regtested Darwin ppc, OK?

2005-11-13  Dale Johannesen  <dalej@apple.com>

        * cfghooks.c (split_block):  Copy loop_father.
        * cfgloopmanip.c (create_loops_notes):  Create preheaders.

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