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/lower/isle.rs

@@ -407,9 +407,15 @@ impl generated_code::Context for IsleContext<'_, '_, MInst, Riscv64Backend> {
             targets,
         });
     }
-    fn x_reg(&mut self, x: u8) -> Reg {
-        x_reg(x as usize)
+
+    fn fp_reg(&mut self) -> PReg {
+        px_reg(8)
     }
+
+    fn sp_reg(&mut self) -> PReg {
+        px_reg(2)
+    }
+
     fn shift_int_to_most_significant(&mut self, v: Reg, ty: Type) -> Reg {
         assert!(ty.is_int() && ty.bits() <= 64);
         if ty == I64 {