[InlineCost] PrinterPass prints constants to which instructions are simplified

This patch enables printing of constants to see which instructions were
constant-folded. Needed for tests and better visiual analysis of
inliner's work.

Reviewers: apilipenko, mtrofin, davidxl, fedor.sergeev

Reviewed By: mtrofin

Differential Revision: https://reviews.llvm.org/D81024