phiopt: New comparison combine part of phiopt [PR126138]

Since phiopt rejects trapping statements, this has to be done
seperately but it does reuse most of the infastructure to handle
this.
This is designed to handle foating point comparisons mergining
with another one.
An example is:
```
c = false;
if (a > b) c = a >= b;
```
This should merge into just `c = a > b;`.
Which we do already if it was written as `a > b && a >= b`.
In this case this is already handled by DOM/VRP/ranger.

The case I am more interesting in is:
```
bool f1(double a, double b)
{
  if (a == b)
    return 1;
  return a > b;
}
```
Which can/should optimize to `a >= b`.
This comes from `(a <=> b) >= 0` without spaceship_replacement and/or
with a patch that forwprops the phi values into the `>= 0`; replacing
the phi.

That is this is prerequisite to
https://inbox.sourceware.org/gcc-patches/20260625233902.2605630-1-andrew.pinski@oss.qualcomm.com/.
and to remove spaceship_replacement in phiopt.

Bootstrapped and tested on x86_64-linux-gnu.

	PR tree-optimization/126138
gcc/ChangeLog:

	* tree-ssa-phiopt.cc (one_feeding_comparison_into_p): New function.
	(comparison_combine): New function.
	(pass_phiopt::execute): Call comparison_combine.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/fp-trapping-cmp-4.c: New test.
	* gcc.dg/tree-ssa/fp-trapping-cmp-5.c: New test.

Signed-off-by: Andrea Pinski <andrew.pinski@oss.qualcomm.com>