[Exegesis][RISCV] Skip some of the tests under expensive checks

Under expensive checks, some of the tests will fail to pass the
MachineVerifier. It's because right after a snippet is generated, its VL
operand (if it's a register) is assigned a physical register. While
we'll replace it with virtual register in RISCVExegesisPreprocessing,
it's technically violating RISCVInstrInfo's validation rule.
Under normal circumstances, this won't trigger a MachineVerifier failure
because the codegen pipeline doesn't validate the code until the very
end -- which is not the case under EXPENSIVE_CHECKS where
MachineVerifierPass is sprinkled here and there.

This is really caused by the fact that RISCV exegesis has an odd
"codegen" Pass pipeline. And I don't have a good solution yet, so I'm
surpressing these tests under EXPENSIVE_CHECKS.