[llvm] [RISCV] Add IsSignExtendingOpW to amocas.w. (PR #79351)

[Craig Topper via llvm-commits]

topperc wrote:

> How come cmpxchg_i32_monotonic_monotonic_val0 already knows it's sign-extending?

SelectionDAG knows atomic ops with i32 memory VT produce sign extended values.

https://github.com/llvm/llvm-project/pull/79351