[Bug rtl-optimization/123822] [15/16 Regression] ICE: in gen_rtx_SUBREG, at emit-rtl.cc:1032 at -Ofast with _Decimal64 vector

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123822

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
There is a missed optimization on the gimple level:
```
  _17 = (<signed-boolean:64>) _16;
...
  _23 = (<signed-boolean:64>) _22;
  _5 = {_17, _23};
  _6 = VEC_COND_EXPR <_5, { -1, -1 }, { 0, 0 }>;
  _25 = BIT_FIELD_REF <_6, 64, 0>;
...
  _28 = BIT_FIELD_REF <_6, 64, 64>;
```

_25 is VCE<_Decimal64>(_16)
_28 is VCE<_Decimal64>(_22)