[Bug tree-optimization/126721] New: UNIFORM_VECTOR_CST[a] -> uniform cst

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126721

            Bug ID: 126721
           Summary: UNIFORM_VECTOR_CST[a] -> uniform cst
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

Take:
```
typedef uint16_t v2u16 __attribute__((vector_size(4)));

int f(int a)
{
    v2u16 bc16={};
    return bc16[a];
}
```

This should just return `0` as bc16 is an uniform vector cst and the values are
all zero. I Noticed this from PR 126720.