A doccomment for CombineTo, and some NFC refactorings

Summary:
Caching SDLoc(N), instead of recreating it in every single
function call, keeps the code denser, and allows to unwrap long lines.

Reviewers: sunfish, atrick, sdmitrouk

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D13726

llvm-svn: 250305