[llvm] [RISCV] Add TSFlag for reading past VL behaviour. NFCI (PR #149704)

[Philip Reames via llvm-commits]

https://github.com/preames commented:

The generic and Rivos custom look fine.  I'd like to have someone from SiFive approve their custom instructions.

https://github.com/llvm/llvm-project/pull/149704