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

This reverts commit 5a7c80ca.  Noticed failures
with the following command:
$ llc -mtriple=riscv64 -mattr=+m,+xtheadba -verify-machineinstrs < test/CodeGen/RISCV/rv64zba.ll

I think I know the cause and will likely reland with a fix tomorrow.