Remove dead code in spirv-tools-opt

- Delete unused LoopValue struct and loop_body_values
- Delete unused LoopInfo fields (_header_label, _header_block_idx, preheader_block_idx)
- Delete unused ternary_op method from context.rs
- Remove type_widths parameter from materialize_term (only_used_in_recursion)
- Fix explicit_counter_loop by using enumerate

Author: LegNeato