[DirectX] add support for i64 buffer load/stores (#145047)
fixes #140321
Specifically it fixes ` error: Cannot create BufferLoad operation:
Invalid overload type`
https://hlsl.godbolt.org/z/dTq4q7o58
but no new DML shaders are building. This change now exposes #144747.
The change does two things it adds i64 support for intrinsic expansion
for the `dx_resource_load_typedbuffer`, and
`dx_resource_store_typedbuffer` intrinsics.
It also lets loaded typedbuffers crash more gracefully because of ` auto
*EVI = cast<ExtractValueInst>(U);` is now a `dyn_cast` and
`llvm_unreachable`.
