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

[Kito Cheng via llvm-commits]

kito-cheng wrote:

I like the direction for moving info into `.td`, I've consider moving `getOperandLog2EEW`/`isSupportedInstr` into `td` file once...but it seems non-trivial work...:P

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