[RISCV] Fix crash when unrolling loop containing vector instructions (#83384)

When MVT is not a vector type, TCK_CodeSize should return an invalid
cost. This patch adds a check in the beginning to make sure all cost
kinds return invalid costs consistently.

Before this patch, TCK_CodeSize returns a valid cost on scalar MVT but
other cost kinds doesn't.

This fixes the issue #83294 where a loop contains vector instructions
and MVT is scalar after type legalization when the vector extension is
not enabled,