[Bug c/99856] New: Alpha Compositing auto vectorization regression: 8.3 -> 9.1

["dushistov at mail dot ru" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99856

            Bug ID: 99856
           Summary: Alpha Compositing auto vectorization regression: 8.3
                    -> 9.1
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dushistov at mail dot ru
  Target Milestone: ---

Such code vectorized with gcc 8.3, but failed to vectorized with 9.1.
Last version of clang works just fine.

```
#include <stdint.h>
#include <stddef.h>

#define SHIFTFORDIV255(a)\
    ((((a) >> 8) + a) >> 8)

#define DIV255(a)\
    SHIFTFORDIV255(a + 0x80)

void
opSourceOver_premul(uint8_t* restrict Rrgba,
                    const uint8_t* restrict Srgba,
                    const uint8_t* restrict Drgba, size_t len)
{
    size_t i = 0;
    for (; i < len*4; i += 4) {
        uint8_t Sa = Srgba[i + 3];
        Rrgba[i + 0] = DIV255(Srgba[i + 0] * 255 + Drgba[i + 0] * (255 - Sa));
        Rrgba[i + 1] = DIV255(Srgba[i + 1] * 255 + Drgba[i + 1] * (255 - Sa));
        Rrgba[i + 2] = DIV255(Srgba[i + 2] * 255 + Drgba[i + 2] * (255 - Sa));
        Rrgba[i + 3] = DIV255(Srgba[i + 3] * 255 + Drgba[i + 3] * (255 - Sa));
    }
}
```

see https://godbolt.org/z/PW65xzb4h
with assembler output for 8.3 and 9.1