[riscv] Fix the disassembler's handling of C.LUI when imm=0

According to the RISC-V spec: "C.LUI is valid only when rd≠{x0,x2},
and when the immediate is not equal to zero. The code points with
imm=0 are reserved".

This change makes the disassembler consider code
points with imm=0 as illegal. It introduces a test which exercises
every C.LUI opcode including the illegal ones but excluding those
assigned to C.ADDI16SP). Output for +c, +c +Zcmop, and +c
+no-rvc-hints is checked.

Author: paulhuggett

llvm/lib/Target/RISCV/Disassembler/RISCVDisassembler.cpp

@@ -453,6 +453,9 @@ static DecodeStatus decodeCLUIImmOperand(MCInst &Inst, uint32_t Imm,
                                          int64_t Address,
                                          const MCDisassembler *Decoder) {
   assert(isUInt<6>(Imm) && "Invalid immediate");
+  if (Imm == 0) {
+    return MCDisassembler::Fail;
+  }
   if (Imm > 31) {
     Imm = (SignExtend64<6>(Imm) & 0xfffff);
   }