disasm: Replace BlockInfo.PairIndex with finer grained indices.
disasm: Replace BlockInfo.PairIndex with finer grained indices.

Replace with BlockInfo.PairIndex four indices:

* IfElseIndex - For an if instruction, this is the index to the
  corresponding else instruction.

* ElseIfIndex - For an else instruction, this is the index to the
  corresponding if instruction.

* EndIndex - For a block/loop/if/else instruction, this is the index
  to the corresponding end instruction.

* BlockStartIndex - For an end instruction, this is the index to the
  instruction that started the block itself.

go-interpreter/wagon,go-interpreter/wagon