This is a pre-req for fixing 80520.  Essentially the goal here is to
keep the key code in this form:

  <bb 3>
  [ ... ]
  if (_20 != 0)
    goto <bb 5>; [50.00%]
  else
    goto <bb 4>; [50.00%]

  <bb 4> [local count: 531502203]:
  _18 = _25 ^ 2567483615;

  <bb 5> [local count: 1063004407]:
  # prephitmp_49 = PHI <_25(3), _18(4)>
  _2 = (void *) ivtmp.8_30;
  MEM[base: _2, offset: 0B] = prephitmp_49;
  ivtmp.8_29 = ivtmp.8_30 + 8;
  if (ivtmp.8_29 != _6)
    goto <bb 3>; [98.99%]
  else
    goto <bb 6>; [1.01%]


Split-paths wants to duplicate bb5 into bb4.  It's just not all that
profitable to do so.  We can get ever-so-slightly better code on a
target like microblaze and perhaps others with delay slots and no
conditional move/execution capabilities.  But that seems more like
something we should be tackling at the RTL level.

To finish fixing 80520 we will need to improve the RTL if-conversion
where we presumably can cost things and make a good choice between the
branchy code we have vs straightline code with a conditional move or
conditional execution.  I'm not tackling that yet.

Note that split-path-5 has the same basic structure.  A half-diamond
with a single statement in the middle block that should be trivially
if-convertable if profitable.  So I adjusted that testcase.

Bootstrapped and regression tested on x86_64.  Installing on the trunk
momentarily.