Assigns the valuetype of the op currently being lowered to a variable
instead of looking it up for each case in the switch statement. This
will make it easier to implement lowering of the new intrinsics for
scalable vector operations in subsequent patches.
Part of the initial SVE codegen series, rfc will be posted soon. Note: this patch is currently just to support the scalable vector codegen rfc, and does not need a full review yet.