[clang] [HLSL][Matrix] Add support for Matrix element and trunc Casts (PR #168915)

[Chris B via cfe-commits]

================
@@ -2422,9 +2422,31 @@ static Value *EmitHLSLElementwiseCast(CodeGenFunction &CGF, LValue SrcVal,
     }
     return V;
   }
+  if (auto *MatTy = DestTy->getAs<ConstantMatrixType>()) {
+    assert(LoadList.size() >= MatTy->getNumElementsFlattened() &&
+           "Flattened type on RHS must have the same number or more elements "
+           "than vector on LHS.");
+
+    llvm::Value *V =
+        CGF.Builder.CreateLoad(CGF.CreateIRTemp(DestTy, "flatcast.tmp"));
+    // write to V.
----------------
llvm-beanz wrote:

```suggestion
    // V is an allocated temporary to build the truncated matrix into.
```

https://github.com/llvm/llvm-project/pull/168915