[clang] [llvm] [HLSL] Reapply Move length support out of the DirectX Backend (#121611) (PR #122337)

[Farzon Lotfi via llvm-commits]

farzonl wrote:

> @llvm-beanz I'm really confused what is going on here The length test case passes in the github action: https://github.com/llvm/llvm-project/actions/runs/12699150463?pr=122337 It passes locally on:
> 
> 1. MacOS Intel (Intel(R) Core(TM) i9-8950HK CPU @ 2.90GHz)
> 2. MacOS Apple M3 Max 12 core
> 3. Ubuntu 22.04 AMD Ryzen Threadripper PRO 5975WX 32-Cores
> 4. Windows AMD Ryzen 9 5900X
> 
> BUT its failing in buildkite and on other llvm builders. I'm not sure what could do this. https://buildkite.com/llvm-project/github-pull-requests/builds/135222

I figured out what is going on here. On some platforms the loop unroll isn't working

This warning shows up:
```
warning: loop not unrolled: the optimizer was unable to perform the requested transformation; the transformation might be disabled or specified as part of an unsupported transformation ordering
```
And there is a `br label %for.body.i`.

I'm switching to define length as `sqrt(dot(x,x))` to avoid the loop.

https://github.com/llvm/llvm-project/pull/122337