Optimize XLA Compiler.
Optimize XLA Compiler.

This results in 2% improvement on some representative large model compilation benchmarks.

1. `MayUseOperandValue(position.index, user)`, `(user->IsRoot() && root_positions.contains(user)` is computed once per <position, user> instead of once per <position, user, operand>. We don't need to pass `i` to `MayUseOperandValue`. We can instead bring the `CHECK` out at the call site.
2. Use `emplace_back` instead of `push_back` and `std::move`. Why: https://godbolt.org/z/sT54Ef88h
