[GlobalDCE] AvailableExternal linkage is checked in isDiscardableIfUnused [NFC].

Summary:
AvailableExternal was not handled in isDiscardableIfUnused when isDiscardableIfUnused
was added in r158476. Till it was handled in r247044. This is a NFC.

Reviewers: pcc, tejohnson

Reviewed By: tejohnson

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D52319

llvm-svn: 342684