[llvm] [RISCV] Handle 'c.addi x0, $imm' alias for c.nop using PseudoC_ADDI_NOP. (PR #150719)

[Sam Elliott via llvm-commits]

lenary wrote:

(I sort of wish the spec didn't say that `c.nop` was a new instruction, just that it was an alias, but there are always going to be edges like this).

https://github.com/llvm/llvm-project/pull/150719