Linker: Avoid scheduling the link of a global value twice due to an alias

3d4f3a0d (https://reviews.llvm.org/D20586) avoided rescheduling
a global value that was materialized first through a regular value, and
then again through an alias. This commit catches the dual, avoiding
rescheduling when the global value is first materialized through an
alias.

Differential Revision: https://reviews.llvm.org/D101419
Radar-Id: rdar://75752728