[llvm] [TTI] Simplify implementation (NFCI) (PR #136674)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon approved this pull request.

LGTM - I love the idea of CRTP, but so much of it always seems to get in the way :( 

https://github.com/llvm/llvm-project/pull/136674