[Bug target/119870] New: `(vector8 unsigned) {0,0x80000000}` and `-0.0` constant formation could be better

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119870

            Bug ID: 119870
           Summary: `(vector8 unsigned) {0,0x80000000}` and `-0.0`
                    constant formation could be better
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---
            Target: aarch64

Take:
```
#define vector8 __attribute__((vector_size(8)))
vector8 unsigned native3(void) {
  double x = -0.0;
  vector8 unsigned t1;
  __builtin_memcpy(&t1, &x, sizeof(x));
  return t1;
}
```

Currently GCC produces:
```
        adrp    x0, .LC0
        ldr     d0, [x0, #:lo12:.LC0]
        ret
```

That is a load from the constant pool.

But we should produce:
```
        movi    d0, #0
        fneg    d0, d0
        ret
```

Note For:
```
double native0(void) {
  double x = -0.0;
  return x;
}
```
GCC produces:
```
        mov     x0, -9223372036854775808
        fmov    d0, x0
        ret
```

Which I also think it might be faster to stay in the fp registers and produce:
```
        movi    d0, #0
        fneg    d0, d0
        ret
```