Preserve the address space for llvm.used and llvm.compiler.used global variables in GlobalOpt pass.

The llvm.used (or llvm.compiler.used) global variable is an array that contains a list of pointers to global variables and functions.

The GlobalOpt (Global Variable Optimizer) pass is not preserving the address space for llvm.used and llvm.compiler.used global variables.This patch updates the setUsedInitializer() function in GlobalOpt.cpp, so the address space is preserved.

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D144518