Clear NewGEPBases after finish using them in CodeGenPrep pass

AFAICT all other set/map are correctly cleared in `runOnFunction`.

With assertion enabled this causes a crash when the module is freed and potentially if a later pass delete the instruction (not observed in real world though). Without assertion this can potentially cause confusing result when running on a new Function/Module.

Reviewed By: loladiro

Differential Revision: https://reviews.llvm.org/D84031