Refactor indirect call promotion profitability analysis (NFC)

Summary:
Refactored the profitability analysis out of the IC promotion pass and
into lib/Analysis so that it can be accessed by the summary index
builder in a follow-on patch to enable IC promotion in ThinLTO (D21932).

Reviewers: davidxl, xur

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D22182

llvm-svn: 275216