Assert that we only use virtual registers with moves (#5440)

Assert that we never see real registers as arguments to move instructions in VCodeBuilder::collect_operands.

Also fix a bug in the riscv64 backend that was discovered by these assertions: the lowerings of get_stack_pointer and get_frame_pointer were using physical registers 8 and 2 directly. The solution was similar to other backends: add a move instruction specifically for moving out of physical registers, whose source operand is opaque to regalloc2.

cranelift/codegen/src/machinst/vcode.rs

@@ -568,11 +568,25 @@ impl<I: VCodeInst> VCodeBuilder<I> {
             }
 
             if let Some((dst, src)) = insn.is_move() {
+                // We should never see non-virtual registers present in move
+                // instructions.
+                assert!(
+                    src.is_virtual(),
+                    "the real register {:?} was used as the source of a move instruction",
+                    src
+                );
+                assert!(
+                    dst.to_reg().is_virtual(),
+                    "the real register {:?} was used as the destination of a move instruction",
+                    dst.to_reg()
+                );
+
                 let src = Operand::reg_use(Self::resolve_vreg_alias_impl(vreg_aliases, src.into()));
                 let dst = Operand::reg_def(Self::resolve_vreg_alias_impl(
                     vreg_aliases,
                     dst.to_reg().into(),
                 ));
+
                 // Note that regalloc2 requires these in (src, dst) order.
                 self.vcode.is_move.insert(InsnIndex::new(i), (src, dst));
             }