[RISCV][GISel] IRTranslate and Legalize some instructions with scalable vector type

* Add IRTranslate tests for ADD, SUB, AND, OR, and XOR with scalable
  vector types to show that they work as expected.
* Legalize G_ADD, G_SUB, G_AND, G_OR, and G_XOR of scalable vector
  type for the RISC-V vector extension.