[llvm] [GlobalISel] Reserve IRTranslator value map and virtual register storage (NFC) (PR #222311)

[Cullen Rhodes via llvm-commits]

c-rhodes wrote:

> Is this also beneficial in cycles? BB.size() is a linked list traversal, probably with a lot of cache misses.

is the cycles metric in the compile-time-tracker reliable? It doesn't show any significant change

https://github.com/llvm/llvm-project/pull/222311