[Bug rtl-optimization/107270] New: [10/11/12/13 Regression] return for structure is not as good as before

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107270

            Bug ID: 107270
           Summary: [10/11/12/13 Regression] return for structure is not
                    as good as before
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---
            Target: aarch64-*-*

Take:
```

struct b
{
  int t;
  int tt;
};

union a
{
  long long t;
  struct b b;
};

struct b f(int i, int t)
{
  return (struct b){i, t};
}

long long f1(int i, int t)
{
  struct b b = {i, t};
  union a a;
  a.b = b;
  return a.t;
}
```
In GCC 8.x, GCC -O2 would produce:
```
f:
        .cfi_startproc
        bfi     x0, x1, 32, 32
        ret
f1:
        .cfi_startproc
        bfi     x0, x1, 32, 32
        ret
```
But in GCC 9 and above GCC produces:
```
f:
        .cfi_startproc
        uxtw    x0, w0
        orr     x0, x0, x1, lsl 32
        ret
f1:
        .cfi_startproc
        uxtw    x0, w0
        orr     x0, x0, x1, lsl 32
        ret
```