cranelift: Drop non-allocatable register operands (#8605)

Consumption of non-allocatable operands was added in #5253 and #5132,
and removed in #8524 and following PRs. Now they are not only ignored by
regalloc2, but the placeholders that it leaves in the allocation results
are also ignored by Cranelift. So let's stop adding them to the operands
list entirely.

cranelift/codegen/src/machinst/reg.rs

@@ -335,11 +335,8 @@ pub trait OperandVisitorImpl: OperandVisitor {
     /// Add a use of a fixed, nonallocatable physical register.
     fn reg_fixed_nonallocatable(&mut self, preg: PReg) {
         self.debug_assert_is_allocatable_preg(preg, false);
-        // Magic VReg which does not participate in register allocation.
+        // Since this operand does not participate in register allocation,
-        let mut reg = Reg(VReg::new(VReg::MAX, preg.class()));
+        // there's nothing to do here.
-        let constraint = OperandConstraint::FixedReg(preg);
-        // This operand won't be allocated, so kind and pos don't matter.
-        self.add_operand(&mut reg, constraint, OperandKind::Use, OperandPos::Early);
     }
 
     /// Add a register use, at the start of the instruction (`Before`