Const correctness for TTI::getRegisterBitWidth

Summary: The method TargetTransformInfo::getRegisterBitWidth() is declared const, but the type erasing implementation classes (TargetTransformInfo::Concept & TargetTransformInfo::Model) that were introduced by Chandler in https://reviews.llvm.org/D7293 do not have the method declared const. This is an NFC to tidy up the const consistency between TTI and its implementation.

Reviewers: chandlerc, rnk, reames

Reviewed By: reames

Subscribers: reames, jfb, arsenm, dschuff, nemanjai, nhaehnle, javed.absar, sbc100, jgravelle-google, llvm-commits

Differential Revision: https://reviews.llvm.org/D33903

llvm-svn: 305189