Auto merge of #147124 - a1phyr:improve_finish_grow, r=Mark-Simulacrum

Move more code to `RawVec::finish_grow`

This move a branch and more code into the cold method `finish_grow`, which means that less code is inlined at each `try_reserve` site. Additionally, this reduces the amount of parameters, so they can all be passed by registers.

Author: bors