[LTO] Don't create a new common unless merged has different size

Summary:
This addresses a regression in common handling from the new LTO
API in r278338. Only create a new common if the size is different.
The type comparison against an array type fails when the size is
different but not an array. GlobalMerge does not handle the
array types as well and we lose some global merging opportunities.

Reviewers: mehdi_amini

Subscribers: junbuml, llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D23955

llvm-svn: 279911