[DirectX] Teach DXILResourceAccess pass to break up large types

[bogner]

Currently DXILResourceAccess doesn't deal well with loads or stores of objects that are bigger than a 4-element vector. We need to break these accesses up into multiple smaller accesses.

[bogner]

I'm having a hard time finding any valid HLSL that can hit issues around this - anything larger than a 4-element vector is already broken up into element-wise accesses by the time we get here. This may change with long vectors and there may be some other case I'm missing, but AFAICT the only way to test this is with fairly artificial IR for now.

[bogner]

I don't believe we need to do anything here, at least with the current structure of DirectX passes - it's impossible to get large accesses past DXILMemIntrinsics, so they never come up. I have added an assert in #173320 that should fire if I'm wrong about this, at which point we can revisit.