[mypyc] Add nested vec irbuild reference counting test

mypyc/test-data/refcount.test

@@ -1867,6 +1867,83 @@ L6:
     dec_ref r2
     goto L2
 
+[case testVecNestedGetItemBorrow_64bit]
+from librt.vecs import vec
+from mypy_extensions import i64
+
+def f(v: vec[vec[i64]], n: i64, m: i64) -> i64:
+    return v[n][m]
+[out]
+def f(v, n, m):
+    v :: vec[vec[i64]]
+    n, m :: i64
+    r0 :: native_int
+    r1 :: bit
+    r2 :: i64
+    r3 :: bit
+    r4 :: bool
+    r5 :: i64
+    r6 :: object
+    r7 :: ptr
+    r8 :: i64
+    r9 :: ptr
+    r10 :: vec[i64]
+    r11 :: native_int
+    r12 :: bit
+    r13 :: i64
+    r14 :: bit
+    r15 :: bool
+    r16 :: i64
+    r17 :: object
+    r18 :: ptr
+    r19 :: i64
+    r20 :: ptr
+    r21 :: i64
+L0:
+    r0 = v.len
+    r1 = n < r0 :: unsigned
+    if r1 goto L4 else goto L1 :: bool
+L1:
+    r2 = n + r0
+    r3 = r2 < r0 :: unsigned
+    if r3 goto L3 else goto L2 :: bool
+L2:
+    r4 = raise IndexError
+    unreachable
+L3:
+    r5 = r2
+    goto L5
+L4:
+    r5 = n
+L5:
+    r6 = v.buf
+    r7 = get_element_ptr r6 items :: VecNestedBufObject
+    r8 = r5 * 16
+    r9 = r7 + r8
+    r10 = borrow load_mem r9 :: vec[i64]*
+    r11 = r10.len
+    r12 = m < r11 :: unsigned
+    if r12 goto L9 else goto L6 :: bool
+L6:
+    r13 = m + r11
+    r14 = r13 < r11 :: unsigned
+    if r14 goto L8 else goto L7 :: bool
+L7:
+    r15 = raise IndexError
+    unreachable
+L8:
+    r16 = r13
+    goto L10
+L9:
+    r16 = m
+L10:
+    r17 = r10.buf
+    r18 = get_element_ptr r17 items :: VecI64BufObject
+    r19 = r16 * 8
+    r20 = r18 + r19
+    r21 = load_mem r20 :: i64*
+    return r21
+
 [case testVecPop]
 from librt.vecs import vec, pop, append
 from mypy_extensions import i64