[RISCV][ISel] Refactor the formation of VW operations

This patch centralizes all the combines of add|sub|mul with extended
operands in one "framework".
The rationale for this change is to offer a one-stop-shop for all these
transformations so that, in the future, it is easier to make combine
decisions for a web of instructions (i.e., instructions connected
through s|zext operands).

Technically this patch is not NFC because the new version is more
powerful than the previous version.
In particular, it diverges in two cases:
- VWMULSU can now also be produced from `mul(splat, zext)`, whereas
  previously only `mul(sext, splat)` were supported when `splat`s were
  involved. (As demonstrated in rvv/fixed-vectors-vwmulsu.ll)
- VWSUB(U) can now also be produced from `sub(splat, ext)`, whereas
  previously only `sub(ext, splat)` were supported when `splat`s were
  involved. (As demonstrated in rvv/fixed-vectors-vwsub.ll)

If we wanted, we could block these transformations to make this
patch really NFC. For instance, we could do something similar to
`AllowSplatInVW_W`, which prevents the combines to form vw(add|sub)(u)_w
when the RHS is a splat.

Regarding the "framework" itself, the bulk of the patch is some
boilderplate code that abstracts away the actual extensions that are
present in the DAG. This allows us to handle `vwadd_w(ext a, b)` as if
it was a regular `add(ext a, ext b)`. Since the node `ext b` doesn't
actually exist in the DAG, we have a bunch of methods (all in the
NodeExtensionHelper class) that fake all that for us.

The other half of the change is around `CombineToTry` and
`CombineResult`. These helper structures respectively:
- Represent the kind of combines that can be applied to a node, and
- Store what needs to happen to do that combine.

This can be viewed as a two step approach:
- First, check if a pattern applies, and
- Second apply it.

The checks and the materialization of the combines are decoupled so that
in the future we can perform several checks and do all the related
applies in one go.

Differential Revision: https://reviews.llvm.org/D134703