[llvm] [RISCV] Use TargetConstant for the immediate RISCVISD::SHL_ADD nodes. (PR #166312)

[Sam Elliott via llvm-commits]

lenary wrote:

Is there a principle for when we should use constants vs target constants? I thought that as RISCVISD nodes were pre-selection, we should be using non-target constants, but I guess that's wrong?

https://github.com/llvm/llvm-project/pull/166312