r206963 - [ARM64] Change inline assembly constraints to be more lax, to match the behaviour of Clang/AArch64 and GCC.

[Eric Christopher]

> +    case 'U':
> +      // Ump: A memory address suitable for ldp/stp in SI, DI, SF and DF modes, whatever they may be
> +      // Utf: A memory address suitable for ldp/stp in TF mode, whatever it may be
> +      // Usa: An absolute symbolic address
> +      // Ush: The high part (bits 32:12) of a pc-relative symbolic address
> +      llvm_unreachable("FIXME: Unimplemented support for bizarre constraints");

Additional review: remove the mode comments - they aren't useful.

-eric