[llvm] [DirectX] Add `llvm.dx.resource.getdimensions.x` intrinsic and lowering to DXIL (PR #161753)

[Tex Riddell via llvm-commits]

================
@@ -746,3 +746,92 @@ Examples:
        @llvm.dx.resource.load.cbufferrow.8(
            target("dx.CBuffer", target("dx.Layout", {i16}, 2, 0)) %buffer,
            i32 %index)
+
+Resource dimensions
+-------------------
+
+*relevant types: Textures and Buffer*
+
+The `getDimensions`_ DXIL operation returns the dimensions of a texture or
+buffer resource. It returns a `Dimensions`_ type, which is a struct
+containing four ``i32`` values. The values in the struct represent the size
+of each dimension of the resource, and when aplicable the number of array
+elements or number of samples. The mapping is defined in the
+`getDimensions`_ documentation.
+
+The LLVM IR representation of this operation has several forms
+depending on the resource type and the specific ``getDimensions`` query.
+The intrinsics return a scalar or anonymous struct with up to 4 `i32`
+elements. The intrinsic names include suffixes to indicate the number of
+elements in the return value. The suffix `.x` indicates a single `i32`
+return value, `.xy` indicates a struct with two `i32` values, and `.xyz`
+indicates a struct with three `i32` values.
+
+Intrinsics representing queries on multisampled texture resources include
+`.ms.` in their name and their return value includes an additional `i32` for
+the number of samples.
+
+Intrinsics with `mip_level` argument and `.level.` in their name are used
+for texture resources with multiple MIP levels. Their return
+struct includes an additional `i32` for the number of levels the resource has.
+
+.. code-block:: llvm
+
+   i32 @llvm.dx.resource.getdimensions.x( target("dx.*") handle )
+   {i32, i32} @llvm.dx.resource.getdimensions.xy( target("dx.*") handle )
+   {i32, i32, i32} @llvm.dx.resource.getdimensions.xyz( target("dx.*") handle )
+   {i32, i32} @llvm.dx.resource.getdimensions.levels.x( target("dx.*") handle, i32 mip_level )
+   {i32, i32, i32} @llvm.dx.resource.getdimensions.levels.xy( target("dx.*") handle, i32 mip_level )
+   {i32, i32, i32, i32} @llvm.dx.resource.getdimensions.levels.xyx( target("dx.*") handle, i32 mip_level )
----------------
tex3d wrote:

typo:
```suggestion
   {i32, i32, i32, i32} @llvm.dx.resource.getdimensions.levels.xyz( target("dx.*") handle, i32 mip_level )
```

https://github.com/llvm/llvm-project/pull/161753