[llvm-branch-commits] [RISC-V][RVY] Add support for compressed stack-pointer addition (PR #203598)

[Alexander Richardson via llvm-branch-commits]

https://github.com/arichardson created https://github.com/llvm/llvm-project/pull/203598

In RVY mode, the c.addi4spn/c.addi16sp instructions expand to YADDI to
perform arithmetic on the YLEN stack pointer instead.
Ideally, we would reuse C_ADDI4SPN/C_ADDI16SP with SP:$rs1, but since the
CompressPat uses instructions with different register classes (ADDI vs YADDI)
we can't do this until tablegen resolves predicates in CompressPat.