[llvm] [ARM] Allow FP reg conversion when copying Sx to Dx (PR #147559)

[via llvm-commits]

eleviant wrote:

> This change seems to break cases that seem entirely legit (although slightly unusual) to me:
> 
> ```c
> #include <arm_neon.h>
> void qt_convert_rgb888_to_rgb32_neon() {
>   register uint8x8_t fullVector asm("d3");
>   asm("" : : "w"(fullVector));
> }
> ```
> 
> Here, `uint8x8_t` and the `w` constraint should be the right ones for the register `d3`.

I will check tomorrow, feel free to revert if it blocks you

https://github.com/llvm/llvm-project/pull/147559