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/isa/riscv64/inst/emit.rs

@@ -1123,6 +1123,19 @@ impl MachInstEmit for Inst {
                     }
                 }
             }
+
+            &Inst::MovFromPReg { rd, rm } => {
+                debug_assert!([px_reg(2), px_reg(8)].contains(&rm));
+                let rd = allocs.next_writable(rd);
+                let x = Inst::AluRRImm12 {
+                    alu_op: AluOPRRI::Ori,
+                    rd,
+                    rs: Reg::from(rm),
+                    imm12: Imm12::zero(),
+                };
+                x.emit(&[], sink, emit_info, state);
+            }
+
             &Inst::BrTable {
                 index,
                 tmp1,