[IRMover] Use GlobalValue::getAddressSpace instead of directly from its type [NFC]

Summary: Change the old form of G->getType()->getAddressSpace() to the new G->getAddressSpace() (underneath does the same).

Patch by Ehud Katz <ehudkatz@gmail.com>

Reviewers: tejohnson, chandlerc

Reviewed By: tejohnson

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69550