disasm: Handle branches to relative depths correctly.
disasm: Handle branches to relative depths correctly.

In WebAssembly, the depth of nested blocks is relative. Therefore,
a branch to depth i will reference the (n-i)th block, where n is the
total number of blocks at that point. This CL fixes the fields in the
operators NewStack so as to reference the correct block.

go-interpreter/wagon,go-interpreter/wagon