[IndVars] Add an option to be able to disable LFTR

This change adds an option disable-lftr to be able to disable Linear Function Test Replace optimization.
By default option is off so current behavior is not changed.

Reviewers: reames, sanjoy, wmi, andreadb, apilipenko
Reviewed By: sanjoy
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D33979

llvm-svn: 305055