Reapply "[RISCV] Implement RISCVISD::SHL_ADD and move patterns into combine (#89263)"

Changes since original commit:
* Rebase over improved test coverage for theadba
* Revert change to use TargetConstant as it appears to prevent the uimm2
  clause from matching in the XTheadBa patterns.
* Fix an order of operands bug in the THeadBa pattern visible in the new
  test coverage.

Original commit message follows:

This implements a RISCV specific version of the SHL_ADD node proposed in
https://github.com/llvm/llvm-project/pull/88791.

If that lands, the infrastructure from this patch should seamlessly
switch over the to generic DAG node. I'm posting this separately because
I've run out of useful multiply strength reduction work to do without
having a way to represent MUL X, 3/5/9 as a single instruction.

The majority of this change is moving two sets of patterns out of
tablgen and into the post-legalize combine. The major reason for this is
that I have an upcoming change which needs to reuse the expansion logic,
but it also helps common up some code between zba and the THeadBa
variants.

On the test changes, there's a couple major categories:
* We chose a different lowering for mul x, 25. The new lowering involves
  one fewer register and the same critical path, so this seems like a win.
* The order of the two multiplies changes in (3,5,9)*(3,5,9) in some
  cases. I don't believe this matters.
* I'm removing the one use restriction on the multiply. This restriction
  doesn't really make sense to me, and the test changes appear positive.

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