PHIOPT: Loop over calling factor_out_conditional_conversion

After adding diamond shaped bb support to factor_out_conditional_conversion,
we can get a case where we have two conversions that needs factored out
and then would have another phiopt happen.
An example is:
```
static inline unsigned long long g(int t)
{
  unsigned t1 = t;
  return t1;
}
unsigned long long f(int c, int d, int e)
{
  unsigned long long t;
  if (c > d)
    t = g(c);
  else
    t = g(d);
  return t;
}
```
In this case we should get a MAX_EXPR in phiopt1 with two casts.
Before this patch, we would just factor out the outer cast and then
wait till phiopt2 to factor out the inner cast.

OK? Bootstrapped and tested on x86_64-linux-gnu with no regressions.

gcc/ChangeLog:

	* tree-ssa-phiopt.cc (pass_phiopt::execute): Loop
	over factor_out_conditional_conversion.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/minmax-17.c: New test.