Reland [NFCI] Refactor X86TargetLowering::getGlobalWrapperKind()

To simplify D150297.

We should be looking at OpFlags more.

Relanding after fix in https://reviews.llvm.org/D159297, hopefully now this is actually NFC.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D157907