Fix unstable SmallPtrSet iteration issues due to collectUsedGlobalVariables

While here, decrease inline element counts from 8 to 4. See D97128 for the choice.

Depends on D97128 (which added a new SmallVecImpl overload for collectUsedGlobalVariables).

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D97139