[clang] [llvm] [HLSL] Implement elementwise popcount (PR #108121)

[Farzon Lotfi via cfe-commits]

================
@@ -0,0 +1,28 @@
+; RUN: opt -S -dxil-op-lower -mtriple=dxil-pc-shadermodel6.3-library %s | FileCheck %s
----------------
farzonl wrote:

so I don't have to clean this up later can you change this to 
```suggestion
; RUN: opt -S -scalarizer -dxil-op-lower -mtriple=dxil-pc-shadermodel6.3-library %s | FileCheck %s
```
And then add a vector test.

https://github.com/llvm/llvm-project/pull/108121