Add several additional ops to the spirv frontend. (#803)

* Add support for int8, int16, and int64 as well as null constant structs

* Add support for OpArrayLength to the spirv frontend

* Add hacky OpCopyMemory support to spirv frontend

* Add OpInBoundsAccessChain to spirv frontend

* Add OpLogical(Not)Equal to spirv frontend

* Fix several of the issues raised in the pr

* Fix the Op::ArrayLength match

* Validate accessing structs through pointers

* Remove extraneous type check