Bug with copy_input both loop_nest and loop_tool

[dejangrubisic]

I was able to save a partially optimized loop_tree, so the agent just needs to label it properly

This is how it looks like:
for m_1677 in 64 : L0
Both LoopTool and LoopNest fail to generate code for copy input operation.
Here is an example and reproducer:

for n_1679 in 16 : L1
for k_1678 in 256 : L2
for m_1677' in 4 : L3
%5[m_1677, k_1678] <- copy(%0)
for n_1679' in 4 : L5
for n_1679'' in 4 : L6
%6[k_1678, n_1679] <- copy(%1)
for m_1677' in 4 : L8
for n_1679' in 4 : L9
for n_1679'' in 4 : L10
%2[m_1677, k_1678, n_1679] <- multiply(%5, %6)
%3[m_1677, n_1679] <- add(%2)
for m_1677' in 4 : L13
for n_1679 in 64 : L14
for n_1679' in 4 : L15
%4[m_1677, n_1679] <- write(%3)

Reproducer:

import loop_tool as lt
import numpy as np
import pdb
def mm(A, B):
    s = lt.SymbolGenerator()
    C = A.to(s.m, s.k) * B.to(s.k, s.n)
    return C.sum(s.k)

m, n, k = 256, 256, 256
A = lt.Tensor(m, k).set(np.random.randn(m, k))
B = lt.Tensor(k, n).set(np.random.randn(k, n))

s = lt.SymbolGenerator()
C = mm(A, B)

loop_tree = C.loop_tree.split(0, 4)\
              .swap_loops(1, 2)\
              .swap_loops(2, 3)\
              .swap_loops(2, 1)\
              .split(1, 16)\
              .swap_loops(2, 3)\
              .swap_loops(3, 4)\
              .copy_input(5, 0)\
              .try_swap(5, 4)\
              .split(5, 4)\
              .copy_input(7, 1)\
              .decrease_reuse(7)\
              .decrease_reuse(7)\
              .decrease_reuse(7)\
              .split(14, 4)

C.set(loop_tree)
with open("data/mm256.txt", "w") as f:
    f.write(C.ir.serialize())

pdb.set_trace()

** Loop Nest
Fails on:

with lt.Backend("loop_nest"):
            mean_runtime = self.tensor.loop_tree.eval()

RuntimeError: assertion: fma_nest failed @ /home/dejang/loop_tool/src/backends/cpu/loop_nest.cpp:30

** Loop Tool
Fails on:
mean_runtime = self.tensor.loop_tree.eval()
error assertion: 0 failed @ /Users/dejang/Desktop/work/loop_tool/src/backends/cpu/cpp.cpp:228 can't emit code for copy