[clang] [llvm] Adding splitdouble HLSL function (PR #109331)

[Farzon Lotfi via llvm-commits]

================
@@ -489,6 +542,9 @@ class OpLowerer {
       case Intrinsic::dx_typedBufferStore:
         HasErrors |= lowerTypedBufferStore(F);
         break;
+      case Intrinsic::dx_splitdouble:
----------------
farzonl wrote:

`replaceFunctionWithNamedStructOp` seems like the right way to go.

My understanding from the meeting was we wanted to define the mapping for `SplitDoubleType` in DXIL.td so we wouldn't need a `case Intrinsic::dx_splitdouble:`?

https://github.com/llvm/llvm-project/pull/109331