[RISCV] Guard the ISD::EXTRACT_VECTOR_ELT handling in ReplaceNodeResults...

[RISCV] Guard the ISD::EXTRACT_VECTOR_ELT handling in ReplaceNodeResults against fixed vectors and non-MVT types.

The type legalizer is calling this code based on the scalar type so
we need to verify the input type is a scalable vector.

The vector type has also not been legalized yet when this is called
so we need to use EVT for it.