[DirectX] Pointer Type Analysis shouldn't use i8* for unknown

[llvm-beanz]

This was an oversight on my part that I just realized. There are cases where the pointer type analysis may not be able to identify the type of a pointer. In those cases today the pointer type analysis emits an i8 and we insert no-op bitcasts in DXILPrepare so that we can emit casts in the final bitcode.

We should probably be using i32 instead of i8 since DXIL doesn't actually have 8-bit types.