[llvm] [RISCV] Add QingKe "XW" compressed opcode extension (PR #97925)

[via llvm-commits]

ArcaneNibble wrote:

@asb you are listed as the code owner for RISC-V

(this is my first time working on LLVM, hopefully I've done things mostly correctly!)

https://github.com/llvm/llvm-project/pull/97925