AD Cleanup: Use a temporary accumulation inst such as IRAccumulate instead of explicitly loading, adding and storing differential values in reverse mode

[saipraveenb25]

We currently accumulate derivatives in a somewhat explicit manner by loading the current differential value, adding the new derivative contribution and then storing this back.

This makes our code fairly messy, and impedes potential optimizations, since accumulation instructions (i.e. +=) can be reordered, but standard loads and stores can't.
We can simply lower these instructions into standard load/add/store instructions after optimizations are performed.
Further, this allows Slang to explicitly emit accumulate instructions on targets that support it (for further downstream optimizations).

Note: IRAccumulate can also have a corresponding IRConsume which would read the derivative and write back a 0. This wouldn't be too helpful other than code clarity, since they can't be reordered. However subsequent consume instructions to the same variable/address can be removed as an optimization (though this is likely to occur anyway with standard loads/stores)

[csyonghe]

This is a good idea and it will keep our IR more compact and this is good for compiler perf as well.