* riscv64: Fix typo in extensions
* riscv64: Move converters to top of file
* riscv64: Group up all imm12 rules
* riscv64: Move zero_reg helpers to Physical Regs section
* riscv64: Move helpers away from `clz` lowerings
These were in the middle of the `clz` rules and are kind of distracting
* riscv64: Move `cls` rules next to `ctz`/`clz`
* cranelift: Move `u8_and` / `u32_add` to Primitive Arithmetic section
* riscv64: Mark some imm12 constructors as pure
* cranelift: Move `s32_add_fallible` next to `u32_add`
* riscv64: Fix Typo