[Bug tree-optimization/126400] New: Improve TARGET_MEM_REF support for load factoring

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126400

            Bug ID: 126400
           Summary: Improve TARGET_MEM_REF support for load factoring
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: pinskia at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---
            Target: aarch64

Created attachment 65128
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=65128&action=edit
testcase

Take the attached testcase at -Os on aarch64.
We get in phiopt4:
```
  if (_7 != 0)
    goto <bb 4>; [50.00%]
  else
    goto <bb 5>; [50.00%]

  <bb 4> [local count: 477815112]:
  _23 = ivtmp.6_20 * 16;
  _6 = MEM[(ix4 *)c_14(D) + _23 * 1];
  goto <bb 6>; [100.00%]

  <bb 5> [local count: 477815112]:
  _24 = ivtmp.6_20 * 16;
  _4 = MEM[(ix4 *)a_13(D) + _24 * 1];

  <bb 6> [local count: 955630224]:
```
Right now the patch which adds support for TARGET_MEM_REF will reject this
because the index is different. But this could be supported.
Adding another phi is doable.