[PartialInlining] Use isInlineViable to detect constructs preventing inlining.

Use isInlineViable to prevent inlining of functions with non-inlinable
constructs, in case cost analysis is skipped.

Reviewers: efriedma, sfertile, davide, davidxl

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D42846

llvm-svn: 327207