Trap reserved vfrdiv.vf encoding as illegal-instruction

[yc199911]

The reserved encoding (funct6=0x21, funct3=1, OPFVV) was decoded
as vfrdiv.vf in op_100001 case 1, executing the FP divide kernel
instead of trapping. Per riscv-opcodes, vfrdiv.vf is defined only
at funct3=5 (OPFVF). Return false in case 1.

---

Summary by cubic

Fix incorrect decode of reserved vfrdiv.vf encoding: OPFVV with funct6=0x21, funct3=1 now returns false to trigger an illegal-instruction trap. Per RISC-V V 1.0, vfrdiv.vf is only valid in OPFVF with funct3=5.

^{Written for commit 8656e04. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}