refactor of visitFADD for AllowNewConst cases

Summary: Refactoring for all constant cases which require AllowNewConst and some staging for future fmf usage.

Reviewers: spatel, hfinkel, wristow

Reviewed By: spatel

Subscribers: nhaehnle

Differential Revision: https://reviews.llvm.org/D48289

llvm-svn: 334984