[RISCV] Assemble/Disassemble v-ext instructions.
Assemble/disassemble RISC-V V extension instructions according to
latest version spec in https://github.com/riscv/riscv-v-spec/.
I have tested this patch using GNU toolchain. The encoding is aligned
to GNU assembler output. In this patch, there is a test case for each
instruction at least.
The V register definition is just for assemble/disassemble. Its type
is not important in this stage. I think it will be reviewed and modified
as we want to do codegen for scalable vector types.
This patch does not include Zvamo, Zvlsseg, and Zvediv.
Differential revision: https://reviews.llvm.org/D69987