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

[Luke Lau via llvm-commits]

lukel97 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

For `isSupportedInstr` there is #149018, and as for `getOperandLog2EEW` I have a rough patch sitting around locally that kinda moves it into tablegen. The idea is that there's only a fixed number of "EEW formats" for each instruction, e.g. EEW=SEW, EEW=SEW*2, etc. So in tablegen there is an enum that contains each format, and each instruction has their format encoded in TSFlags

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