[TTI] Constify BasicTTIImplBase::thisT() (NFCI) (#136575)

The main change is making `thisT` method `const`, the rest of the
changes is fixing compilation errors (*).

(*) There are two tricky methods, `getVectorInstrCost()` and
`getIntImmCost()`.
They have several overloads; some of these overloads are typically
pulled in to derived classes using the `using` directive, and then
hidden by methods in the derived class.
The compiler does not complain if the hiding methods are not marked as
`const`, which means that clients will use the methods from the base
class. If after this change your target fails cost model tests, this
must be the reason. To resolve the issue you need  to make all hiding
overloads `const`. See the second commit in this PR.

Pull Request: https://github.com/llvm/llvm-project/pull/136575

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