Use IDifferentiablePtrType for function-scope vars that cannot be SSA'd for performance reasons

[saipraveenb25]

We currently have a special SSA pass that converts all types (arrays, structs, structs of structs, etc..) into SSA types and converts reads and writes into OpGetElement/OpUpdateElement instructions with access-chain information.

This was initially done in order to avoid dealing with OpVar and OpPtrType instructions. However, it makes sense to treat these as 'buffer' types (at a function-local level) and treat loads and stores from these as we would from a resource object (turn loads into "store & set-zero" and stores into "atomic accumulates").

This avoid some catastrophic situations where aggregate objects that are modified within a loop will cause the entire aggregate to be stored per-iteration (unnecessarily), causing the resulting kernel to be far too slow, or refuse to compile entirely

This issue will be easier to resolve once the IDifferentiablePtrType system is implemented (Issue #4998), as we can express PtrType<T : IDifferentiable> as an IDifferentiablePtrType
This issue also depends on #4211 (which implements an intermedate OpAccumulate inst that stands in for the OpLoad/OpCall("dadd")/OpStore combo that causes a ton of clutter in our reverse-mode pass)