[xla] hlo_computation: drop instruction_indices_
[xla] hlo_computation: drop instruction_indices_

It's redundant given that we have HloInstruction.index_in_parent_.
There is only one caller (a TF_RET_CHECK) that needs it; I think
we can survive without that check.

This reduces the size of the struct from 176 to 152 bytes.

While at it, move to_be_deleted_ up in the struct so that
instructions_, instruction_count_ and to_be_deleted_ are now
contiguous.
