When the list is large, using `llvm::is_contained` is of higher
performance than a sequence of comparisons. When the list is small,
the `llvm::is_contained` can be inlined and unrolled, which has the
same effect as using a sequence of comparisons.

And the generated code is more readable.

Change `getValueAsListOfDefs` to return a vector of const Record
pointer, and remove `getValueAsListOfConstDefs` that was added as a
transition aid.

This is a part of effort to have better const correctness in TableGen
backends:


https://discourse.llvm.org/t/psa-planned-changes-to-tablegen-getallderiveddefinitions-api-potential-downstream-breakages/81089

Adopt scaled indent in PredicateExpander.
Added pre/post inc/dec operators to `indent` and related unit tests.
Verified by comparing *.inc files generated by LLVM build with/without
the change.

Use const Record pointers in PredicateExpander.

This is a part of effort to have better const correctness in TableGen
backends:


https://discourse.llvm.org/t/psa-planned-changes-to-tablegen-getallderiveddefinitions-api-potential-downstream-breakages/81089

Refactor of the llvm-tblgen source into:
- a "Basic" library, which contains the bare minimum utilities to build
`llvm-min-tablegen`
- a "Common" library which contains all of the helpers for TableGen
backends. Such helpers can be shared by more than one backend, and even
unit tested (e.g. CodeExpander is, maybe we can add more over time)

Fixes #80647