[llvm] [RISCV][VLOPT] Add vsadd and vsaddu to isSupportedInstr (PR #120358)

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

Had a chat offline about this with @topperc and @kito-cheng. Going to close this because of VXSAT. No other VXSAT writing instructions are already in isSupportedInstr.

https://github.com/llvm/llvm-project/pull/120358