Enable EliminateAvailableExternally pass for -O1

Summary:
Follow-up to D43690, the EliminateAvailableExternally pass currently
runs under -O0 and -O2 and up. Under -O1 we would still want to drop
available_externally symbols to reduce space without inlining having
run.

Reviewers: tejohnson

Reviewed By: tejohnson

Subscribers: mehdi_amini, llvm-commits, kcc

Differential Revision: https://reviews.llvm.org/D46093

llvm-svn: 330961