[llvm] [RISCV] Use isCompatible when we need runtime VSETVLIInfo equality. NFC (PR #94340)

[Luke Lau via llvm-commits]

lukel97 wrote:

I've tweaked this to reuse isCompatible instead of introducing a new method. == should now be strictly for lattice equality

https://github.com/llvm/llvm-project/pull/94340