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

[Luke Lau via llvm-commits]

lukel97 wrote:

Turns out we shouldn't use it in intersect, as that still requires lattice equality. We also don't need it for needVSETVLIPHI since we bail out for the case without a VNInfo in #94686 

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