[LV] Simplify register usage code and avoid double map lookups. NFC

Instead of checking whether a map entry exists to decide if we should
initialize it or add to it, we can rely on the map entry being constructed
and initialized to 0 before the addition happens.

For the std::max case, I've made a reference to the map entry to
avoid looking it up twice.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D135977