[Bug target/112384] a non-constant vec dup should be improved

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112384

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Oh f2 just goes to memory.
Produces:
```
        and     x0, x0, 3
        str     q0, [sp]
        ldr     s0, [sp, x0, lsl 2]

        dup     v0.4s, v0.s[0]
```

Now clang(LLVM) produces:
```
        mov     x8, sp
        and     w9, w0, #0x3
        str     q0, [sp]
        orr     x8, x8, x9, lsl #2
        ld1r    { v0.4s }, [x8]
```

I don't know which is better but it might be the case where GCC's is better for
some micro-arch.