Revert "Add fixed-non-allocatable operand support."

This feature needs more thought; for now we will of course continue to
support pinned vregs, but perhaps we can do better for
"pass-through-and-forget" operands that are given non-allocatable
registers.

This reverts commit 736f636c36.

src/ion/dump.rs

@@ -1,7 +1,7 @@
 //! Debugging output.
 
 use super::Env;
-use crate::{Block, Function, ProgPoint};
+use crate::{Function, ProgPoint, Block};
 
 impl<'a, F: Function> Env<'a, F> {
     pub fn dump_state(&self) {

src/ion/liveranges.rs

@@ -272,9 +272,6 @@ impl<'a, F: Function> Env<'a, F> {
 
                 for pos in &[OperandPos::After, OperandPos::Before] {
                     for op in self.func.inst_operands(inst) {
-                        if op.as_fixed().is_some() {
-                            continue;
-                        }
                         if op.pos() == *pos {
                             let was_live = live.get(op.vreg().vreg());
                             log::debug!("op {:?} was_live = {}", op, was_live);
@@ -871,15 +868,6 @@ impl<'a, F: Function> Env<'a, F> {
                             operand
                         );
 
-                        // If this is a "fixed non-allocatable
-                        // register" operand, set the alloc
-                        // immediately and then ignore the operand
-                        // hereafter.
-                        if let Some(preg) = operand.as_fixed() {
-                            self.set_alloc(inst, i, Allocation::reg(preg));
-                            continue;
-                        }
-
                         match operand.kind() {
                             OperandKind::Def | OperandKind::Mod => {
                                 log::debug!("Def of {} at {:?}", operand.vreg(), pos);