[DirectX] Dot4AddU8Packed and Dot4AddI8Packed Do not match validator expectations

[farzonl]

We are seeing many errors for this:

7680x error: 'dx.op.dot4AddPacked' is not a DXILOpFuncition for DXILOpcode 'Dot4AddU8Packed'.
7680x error: 'dx.op.dot4AddPacked' is not a DXILOpFuncition for DXILOpcode 'Dot4AddI8Packed'.
48x error: External function 'dx.op.dot4AddPacked.i32' is unused.

The fix seems to be to set the result type to OverloadTy see: 90a97c4