[xla] Annotate called computations with instruction type after flattening call graph
[xla] Annotate called computations with instruction type after flattening call graph

Each HloComputation has a field that tells to what kind of HloInstruction it is attached to. It is lost when we clone computations to flatten the graph. Also it can be lost in earlier rewrites.

After call graph flattening it is guaranteed that HloComputation <-> HloInstruction has a 1 to 1 mapping and each computation has exactly one callee instruction.

This information is required for later rewrites.
