Revert "get rid of instruction set specific invalid instructions",
Revert "get rid of instruction set specific invalid instructions",

because Clash didn't like this change.

This reverts commit e22c76a71a8f7828be5b7b9468e58fd64728634d.

mit-plv/riscv-semantics,mit-plv/riscv-semantics,mit-plv/riscv-semantics,mit-plv/riscv-semantics,mit-plv/riscv-semantics