[NFC] Uniquify 'const' in TargetTransformInfoImpl.h

Some member functions of class TargetTransformInfoImplBase in
TargetTransformInfoImpl.h are marked const while others are not. Yet all
of the should be marked const since they are just providing default TTI
values. This patch fixes the inconsistency.

Authored-by: Jinzheng Tu <b1f6c1c4@gmail.com>

Reviewed By: simoll

Differential revision: https://reviews.llvm.org/D93573