FFI: allow to choose not to emit ffi calls in IR

[hzeller]

Right now, if there is an FFI call annotation #[extern_verilog(...)] on a dslx function, it is always emitted to IR, is skipped in global optimization in opt-ir and ultimately emitted in codegen.

It would be good to have a flag in the ir-conversion and optimizer to ignore ffi calls and use the underlying dslx-IR. This allows a/b comparisons.