[mypyc] Add LoadGlobal IR (#8973)
In this PR we introduce a LoadGlobal IR that loads a name in global space.
It's part of mypyc/mypyc#736 and mypyc/mypyc#737, it replaces literal
LoadStatic with LoadGlobal and moves the name generation logic to caller
in irbuild. It will eventually replace LoadStatic.
LoadGlobal simply reads a global value via a given name (its counterpart for
reading an address will be a future LoadAddress).
This PR also introduces a registry and a description for making LoadGlobal
work with more customized loading ops (some name_ref_ops like the True
and False).
