[RISCV] Teach ComputeNumSignBitsForTargetNode about masked atomic intrinsics

[RISCV] Teach ComputeNumSignBitsForTargetNode about masked atomic intrinsics

An unnecessary sext.w is generated when masking the result of the
riscv_masked_cmpxchg_i64 intrinsic. Implementing handling of the
intrinsic in ComputeNumSignBitsForTargetNode allows it to be removed.

Although this isn't a particularly important optimisation, removing the
sext.w simplifies implementation of an additional cmpxchg-related
optimisation in D130192.

Although I can't produce a test with different codegen for the other
atomics intrinsics, these are added as well for completeness.

Differential Revision: https://reviews.llvm.org/D130191