CircRNA peptide nodes with and without a silent mutation should not be collapsed

[zhuchcn]

Found by fuzz test.

Consider a peptide variant graph below. The mutation 11-G/C is missense in the first loop and is a silent mutation in the second loop. The circRNA is so small that only has a very small exon.

    A     G
   / \   / \
MQV-G-HAG-G-TCR

After aligning to the cleavage sites, there are two pairs of nodes that have identical sequence. Then if we collapse the nodes with the same sequence, we will lose the varient peptide sequence.

       MQVGHAGGTCR
      /           \
[root]-MQVGHAGGTCR-[downstream]
     |\           /|
     | MQVAHAGGTCR |
      \           /
       MQVAHAGGTCR