[Bug target/114848] New: longarch: epilogue in _Unwind_RaiseException corrupts return value due to __builtin_eh_return

["pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114848

            Bug ID: 114848
           Summary: longarch: epilogue in _Unwind_RaiseException corrupts
                    return value due to __builtin_eh_return
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---
            Target: longarch64-linux-gnu

I reduced an miscompile for aarch64 inside _Unwind_RaiseException and I noticed
the same issue can be reproduced on powerpc64-linux-gnu and powerpc-linux-gnu.

Reduced testcase:
```
__attribute__((noipa,noinline))
int f(int *a, long offset, void *handler)
{
  if (*a == 5)
    return 5;
  __builtin_eh_return (offset, handler);
}

int main()
{
  int t = 5;
  if (f(&t, 0, 0) != 5)
    __builtin_abort();
}
```
This produces a load (of r4) in the epilogue part for the `return 5` path
(which is reduced from the end of stack path inside _Unwind_RaiseException).


```

        bne     $r13,$r12,.L2
        addi.w  $r4,$r0,5             # 0x5
        or      $r8,$r0,$r0
.L4:
        ld.d    $r1,$r3,40
        .cfi_remember_state
        .cfi_restore 1
        ld.d    $r5,$r3,24
        .cfi_restore 5
        ld.d    $r4,$r3,32
        .cfi_restore 4
        ld.d    $r6,$r3,16
        .cfi_restore 6
        ld.d    $r7,$r3,8
        .cfi_restore 7
        addi.d  $r3,$r3,48
        .cfi_def_cfa_offset 0
        add.d   $r3,$r3,$r8
        jr      $r1
```