[Assignment Tracking] Initialise maps with minimum required number of entries

The size lower bound is known - the `Join` map in both cases gets an entry for
each variable from both input maps (union).

This reduces the number of times the map grows, improving ReleaseLTO-g compile
time for CTMark projects by an average of around 0.2%.

Reviewed By: scott.linder

Differential Revision: https://reviews.llvm.org/D144486