[llvm] [RISCV] Move slide and gather costing to subtarget [NFC] (PR #65396)

[Craig Topper via llvm-commits]

topperc wrote:

> > Where was this discussed in D159332?
> > "Discussed" was the wrong word.  It was one of the TODOs which had gone without discussion.
> > Why do we want this in Subtarget? Seems weird to start bring InstructionCost into Subtarget.
> > We talked offline, and both felt subtarget wasn't the right place for this.  We were tentatively thinking putting this on TLI.  Is that still what you're thinking?

Yes. That's the best place I can think of right now.

https://github.com/llvm/llvm-project/pull/65396