[Exegesis][RISCV] Add initial RVV support (#128767)
This patch adds initial vector extension support to RISC-V's exegesis.
The strategy here is to enumerate all RVV _pseudo_ opcodes as their MC
opcode counterparts are kind of useless under this circumstance. We also
enumerate all possible VTYPE operands in each CodeTemplate
configuration. Various of MachineFunction Passes are used for post
processing the snippets, like inserting VSETVLI instructions.
See https://llvm.org/devmtg/2024-10/slides/techtalk/Hsu-RVV-Exegesis.pdf
for more technical details.
