[Bug target/106327] New: side-effect-free _x variance not optimized to unpredicated instruction

["yyc1992 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106327

            Bug ID: 106327
           Summary: side-effect-free _x variance not optimized to
                    unpredicated instruction
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: yyc1992 at gmail dot com
  Target Milestone: ---

Related to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106326 .

According to the Arm C Language Extension for SVE, when the _x predicate is
used,

> The compiler can then pick whichever form of instruction seems to give the best code. This includes using unpredicated instructions, where available and suitable

Because of this, I'm expecting the following to be optimized to a single add
instruction, as if a `svptrue_b64()` predicate is used.

```
svfloat64_t add(svfloat64_t a, svfloat64_t b)
{
    auto und_ok = svcmpge(svptrue_b64(), a, b);
    return svadd_x(und_ok, a, b);
}
```

However, gcc compiles this as _m and generates

```
        ptrue   p0.b, all
        fcmge   p0.d, p0/z, z0.d, z1.d
        fadd    z0.d, p0/m, z0.d, z1.d
```

In general, is there any reason not to treat an `add_x` (also other
side-effect-free functions) with an unknown predicate as unpredicated one?