[llvm] [DirectX] Support the CBufferLoadLegacy operation (PR #128699)

[Justin Bogner via llvm-commits]

================
@@ -46,6 +46,12 @@ def ResRetDoubleTy : DXILOpParamType;
 def ResRetInt16Ty : DXILOpParamType;
 def ResRetInt32Ty : DXILOpParamType;
 def ResRetInt64Ty : DXILOpParamType;
+def CBufRetHalfTy : DXILOpParamType;
----------------
bogner wrote:

`dx.op.cbufferLoadLegacy` returns a named structure containing elements that are half, float, double, int16, int32, or int64, but it names its overload based on the element type. So we have these types:
```llvm
; 4 32-bit types
%dx.types.CBufRet.f32 = type { float, float, float, float }
%dx.types.CBufRet.i32 = type { i32, i32, i32, i32 }
; 2 64-bit types
%dx.types.CBufRet.f64 = type { double, double }
%dx.types.CBufRet.i64 = type { i64, i64 }
; 8 16-bit types (If -enable-16bit-types is present)
%dx.types.CBufRet.f16.8 = type { half, half, half, half, half, half, half, half }
%dx.types.CBufRet.i16.8 = type { i16, i16, i16, i16, i16, i16, i16, i16 }
```

and we have these overloads:
```llvm
declare %dx.types.CBufRet.f32 @dx.op.cbufferLoadLegacy.f32(...)
declare %dx.types.CBufRet.i32 @dx.op.cbufferLoadLegacy.i32(...)
declare %dx.types.CBufRet.f64 @dx.op.cbufferLoadLegacy.f64(...)
declare %dx.types.CBufRet.i64 @dx.op.cbufferLoadLegacy.i64(...)
declare %dx.types.CBufRet.f16 @dx.op.cbufferLoadLegacy.f16(...)
declare %dx.types.CBufRet.i16 @dx.op.cbufferLoadLegacy.i16(...)
```

The DXILEmitter association is telling us what to name the overloads, and the overloads are cbuffer types in DXIL.td so that we return these structs and not, say, a single `half`.

https://github.com/llvm/llvm-project/pull/128699