[ThinLTO] Avoid extra index lookup during promotion

Summary:
Pass down the already accessed ValueInfo to shouldPromoteLocalToGlobal,
to avoid an unnecessary extra index lookup.

Add some assertion checking to confirm we have a non-empty VI when
expected.

Also some misc cleanup, merging the two versions of
doImportAsDefinition, since one was only called by the other, and
unnecessarily passed in a member variable.

Reviewers: steven_wu, pcc, evgeny777

Reviewed By: evgeny777

Subscribers: mehdi_amini, inglorion, hiraditya, dexonsmith, arphaman, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70337