[llvm] [RISCV] Implement RISCVInstrInfo::isAddImmediate (PR #72356)

[Alex Bradbury via llvm-commits]

asb wrote:

> Note that although the doc comment for isAddImmediate specifies its behaviuor solely in terms of physical registers, none of the current in-tree implementations (including this one) bail out on virtual registers.

I've posted a doc change PR #72357 to adjust this (or for someone to tell me why that's the wrong thing to do!).

https://github.com/llvm/llvm-project/pull/72356